Azurill is obtainable on Routes 3 and 22 as a relatively uncommon encounter. Marill can be obtained through Surf on Route 3 as well, at a 33.3% encounter rate. As well as these, Azumarill can be obtained on Route 22 as a rare surfing encounter and it's also available in the yellow flowers as a relatively common one.
The Azurill line is one of those Pokémon families that have received plenty of buffs over the generations, going from an unremarkable tank in GSC, to a destroyer with an ability that pushes its attack to rival those of Groudon and Zekrom. These Pokémon once more got gifted in the first 3D handheld outing for the main series, gaining the fancy new Fairy-type and a small boost to Azumarill's special attack. Adding to this, due to mechanic changes, members of this line caught as Azurill will be certain to have at least three perfect IVs. As a result, the Azumarill line, regardless of ability, is a force to be reckoned with in Kalos. If only Play Rough came at level 25 like in ORAS, Azumarill would be nearly unstoppable, although this Pokémon still comes pretty close.
Gym #1 - Viola (Santalune City, Bug-type): The line's new found resistance to Bug-type moves will come in handy here, reducing the threat posed by Vivillon's Infestation. Marill does significantly better than Azurill here, due to its higher stats and resistance to Surskit's Bubble. Click Rollout and watch the chaos ensue.
Professor Sycamore (Lumiose City): Charmander does not stand up to STAB Bubblebeam, even with Marill's questionable special attack. Squirtle is a slower prospect, however, Marill should probably come out on top through Return. Bulbasaur is a no-go due to type match up. Azurill is too weak to handle any of the Kanto Starters.
Tierno (Route 5): Corphish has Swords Dance and a high attack stat. This is not Marill's fight.
Tierno and Trevor (Route 7, tag battle with rival): Avoid Pikachu for obvious reasons, and exert caution around Tierno's Corphish. However, Flabébé should not present much of a threat to an Azumarill, even with Vine Whip.
Gym #2 - Grant (Cyllage City, Rock-type): Azumarill is a pretty solid check here, as result of its resistance to his Pokémon's coverage option,s and being able to hit them with STAB Aqua Tail. Just be careful around Tyrunt, since it hits hard with Rock Tomb.
Korrina (Geosenge Town, Fighting-type): Slap the two Lucario around with Aqua Tail, Azumarill's new typing is a wonderful addition for dealing with these Pokémon.
Rival (Tower of Mastery): Out of their potential Pokémon, the only one Azumarill legitimately wants nothing to do with is Quilladin, as a result of its bulk and Grass-typing. Everything else can be handled between Aqua Tail and Return.
Gym #3 - Korrina (Shalour City, Fighting-type): While Azumarill is unable to just destroy Korrina's Pokémon with Play Rough, on the account of it being about 20 levels away, they are unlikely to be doing much back in return. Apply ample servings of Aqua Tail and this should be a short one.
Rival (Coumarine City): This is literally a higher level repeat of the previous fight. Once more, the one Pokémon Azumarill wants to avoid is Quilladin. Everything else is a free target, go get them.
Gym #4 - Ramos (Coumarine City, Grass-type): Too many Grass-type Pokémon for Azumarill to be comfortable here, even if its light weight means Grass Knot is not as powerful as it could be. Weepinbell is more of a threat than normal due to Poison being strong against Fairy-types, one of the few times this upgrade backfires on Azumarill. However, those with Huge Power are able to 2HKO it with Return, in contrast to Grass Knot's 3HKO. It is a similar story with Jumpluff, but Gogoat is too bulky for even Huge Power variants to bring it down first in the face of a 2HKO from Grass Knot.
Aliana (Kalos Power Plant): The main issue here is Mightyena's Intimidate. However, Swagger, which it is likely to use, undoes this, essentially giving Azumarill free reign to run over it with Aqua Tail or Power Up Punch.
Gym #5 - Clemont (Lumiose City, Electric-type): Nope. Take everything bad about Ramos and replace Grass Knot with Thunderbolt on Pokémon with high special attack stats.
Rival (Route 14): As before, Azumarill can handle Absol without issue. By contrast, Meowstic might prove to be dangerous to Thick Fat Azumarill because of Light Screen and STAB Psychic, but Huge Power ones should crack it with care. Greninja has no way to deal anything close to relevant damage, while Delphox can be risky through STAB Psybeam, but then Aqua Tail destroys it. Chesnaught's Seed Bomb is enough of a deterrent to our aquatic bunny, let alone its beefy physical stats.
Gym #6 - Valerie (Laverre City, Fairy-type): While her Mawile may lack a way to deal with Azumarill, Iron Defense can result in this being a drawn out pain of a fight, given that it allows Mawile to boost faster than Azumarill can with Power Up Punch. However, nothing is stopping you from setting up to plus 6 on it. From there, just be ready to heal when needed and Mr. Mime and Sylveon should go down without too much trouble, so enjoy your new TM99. Alas, Azumarill does not learn Dazzling Gleam sadly.
Celosia and Bryony (Poké Ball Factory, tag battle with rival): Manectric is a fast Electric-type with a STAB move. Best to give it a miss, although if you can get the 2v1 with Liepard, Azumarill can put in a shift here.
Mable (Frost Cavern): Aqua Tail OHKO, GG.
Rival (Anistar City): Absol and Meowstic are the same as before, expect now there is a chance you have Play Rough at your disposal. As an aside for non-Huge Power members of the line, the cat no longer has Light Screen to slow you down. They now bring an eeveelution to the table. Flareon is a joke, while Jolteon should be avoided, regardless of ability, while Vaporeon is best handled by those with Huge Power, as Grass Knot will take a while to KO. In terms of their starter, Greninja still rolls over without doing much, while Delphox will manage to take a chuck out of your Pokémon through STAB Psychic, but Aqua Tail will KO it with ease. Avoid Chesnaught at all costs, even if you have Play Rough.
Gym #7 - Olympia (Anistar City, Psychic-type): Azumarill's low speed comes to be a problem here, as Sigilyph will be able to get up Reflect before it is taken down. Under Reflect, Slowking is too bulky for Azumarill to deal with, and it will be able to take your Pokémon down after a few Calm Minds. Meowstic is the same as the previous ones, only this one has Calm Mind, which can be a major threat to any Azumarill without Huge Power. Spamming X-attacks to wait out Reflect is a potential option that can make this fight significantly easier for physically focused Azumarill.
Lysandre battle #1 (Lysandre Labs): Azumarill can take out everything but the Gyarados through its STAB moves alone. Gyarados is too bulky and resists Azumarill's main coverage option, Superpower, on top of Intimidate slashing through your Pokémon's attack stat. Send a fast Electric-type in to deal with this, just don't hard switch in, due of Earthquake.
Aliana (Lysandre Labs): Power Up Punch on the Mightyena to undo Intimidate and then blow Druddigon away with Play Rough.
Celosia (Lysandre Labs): Manectric has STAB Thunderbolt and Drapion has Poison Jab to kill Azumarill with; this matchup is not worth fighting.
Bryony (Lysandre Labs): Liepard is not a threat in the slightest, and Superpower effortlessly OHKOs Bisharp.
Mable (Lysandre Labs): Hey look, more Dark-types for Azumarill to exploit its new resistance and super effective STAB against.
Xerosic (Lysandre Labs): Stay away from Crobat, high speed and a super effective STAB move is not a fun combination to deal with. By contrast Malamar is significantly more manageable, particularly for those Azumarill with Huge Power, earning a 2HKO with Play Rough, with a chance at a OHKO. Those with Thick Fat have to settle with a 3HKO and need to be careful of it setting up with Superpower, which makes Psycho Cut, normally a 4HKO significantly more dangerous.
Lysandre battle #2 (Team Flare Secret HQ): Azumarill can take out everything but the Gyarados through its STAB moves alone. Gyarados is too bulky and resists Azumarill's main coverage option in Superpower on top of Intimidate slashing through your Pokémon's attack stat. Send a fast Electric-type in to deal with this, just don't hard switch in because of Earthquake.
Xerneas (Team Flare Secret HQ, X only): Azumarill is not getting this thing down to the point where basic Pokéballs would be reliable before it gets a Geomancy off, and then tears through whatever you try and send in... if it didn't have a relatively high catch rate meaning that even at full, a Quick Ball is likely to be enough to do it turn 1. Plus, you have the Master Ball, and this fight repeats until you catch it.
Yveltal (Team Flare Secret HQ, Y only): Yveltal is significantly more manageable for Azumarill, but the above points about catching Xerneas ring true here as well.
Lysandre battle #3 (Team Flare Secret HQ): Azumarill can take out everything on his team through its STAB moves alone, although it cannot beat Mega-Gyarados one on one, as a result of it having Intimidate prior to Mega-evolving. However, Azumarill's typing does make it a valid switch-in to Gyarados' options. Just keep an eye on the aqua rabbit's health and it should be fine.
Professor Sycamore (Couriway Town): Charizard and Blastoise are easy pickings for Azumarill; Venusaur, on the other hand, is to be avoided.
Shauna (Route 19): Delcatty may not be a threat in terms of damage output, but Charm can be annoying, nothing Power Up Punch cannot undo, however. Surprisingly, Shauna actually decided to give her Goodra anti-fairy coverage in the form of Sludge Wave, making this engagement inadvisable given Azumarill's low speed and Goodra's general bulk. The starters are pretty much the same as with the previous rival fight in terms of what Azumarill can do here.
Tierno (Route 19): This fight is a joke, just stay from Roserade, but Crawdaunt and Talonflame should not be a threat whatsoever.
Trevor (Route 19): Raichu should be avoided for obvious reasons, namely STAB Thunderbolt, while Florges is far too bulky for special attacking Azumarill to break through, with Energy Ball and Confide forcing your Pokémon out. The former is still a threat for Huge Power variants, but Aqua Tail should be enough, given the level gap that should be a thing since Lysandre was stronger then this lot. Aerodactyl is doable regardless.
Gym #8 - Wulfric (Snowbelle City, Ice-type): Abomasnow is a Grass-type, do not bother, even if Azumarill has Superpower. Cryogonal, on the other hand, is easy pickings, just be wary of Confuse Ray. Avalugg is a brick wall that Azumarill will have troubles taking down without setting up. However, after a few Power Up Punches, Azumarill should be able to KO the titan with Superpower.
Rival (Victory Road): Outside of their new Altaria, this is pretty much a higher leveled repeat of the Anistar City fight. Speaking of said new member, Ice Beam or Play Rough will bring it down without much issue.
Elite Four Wikstrom (Pokémon League, Ironworks Chamber, Steel-type): Azumarill can do stuff here, but lacking the resistance to Steel as a result of its Fairy subtyping is problematic against the likes of Technician Scizor and Aegislash, with King Shield crippling Azumarill for making contact damage. Probopass is probably doable, in spite of it having Discharge, however.
Elite Four Malva (Pokémon League, Blazing Chamber, Fire-type): Aqua Tail/Waterfall/Surf everything and you win, just keep an eye on Azumarill's health as her Pokémon all have ways to hit Water-types neutrally, and STAB Shadow Balls/Brave Birds from Chandelure/Talonflame hurt, even when neutral.
Elite Four Drasna (Pokémon League, Dragonmark Chamber, Dragon-type): The only thing Azumarill cannot deal with here is Dragalge due to its Poison-typing, everything else falls to Play Rough or Ice Beam.
Elite Four Siebold (Pokémon League, Flood Chamber, Water-type): You can beat up Clawitzer as it lacks a way to do neutral damage to Azumarill. Likewise, Barbaracle will fall to a Power Up Punch followed by Superpower without much hassle, as Stone Edge is a 3HKO. Gyarados is best avoided as it can and will use Azumarill as an chance to set up with Dragon Dance, which is not a fun experience to deal with. However, Starmie can only manage a 3HKO with Psychic at best, while Play Rough is a 2HKO, just switch in the event of it proccing the stat drop.
Champion Diantha (Pokémon League, Radiant Chamber): Hawlucha may have Poison Jab to try and deter Fairy-types from mauling it, however, without using Swords Dance first, it is only a 3HKO, in contrast to Play Rough's 1HKO (even after the dance, should Play Rough miss, it is still only a 2HKO). Tyrantrum's Head Smash hits like a freight train, shaving off about 75% of a level 67 Azumarill's HP. However, the recoil that it suffers in response combined with Play Rough should be enough to bring it down in one turn. Aurorus is fast enough to avoid being destroyed by Superpower with Reflect, and Thunder does between 54 and 64% to Azumarill. However, while this fight is probably impossible without Thunder missing, had Azumarill come off fighting Tyrantrum without being healed, it is a manageable experience if your Pokémon is fresh. Avoid Gourgeist as STAB super effective Seed Bomb is not pleasant to endure, and Azumarill does not have much to hit back with. Goodra is an easy target, lacking any method to do neutral damage to Azumarill here. In regards to her mega, Azumarill wants little to do with Mega-Gardevoir without boosts, as Play Rough/Aqua Tail only manage to 4HKO at best without being over leveled, while its Thunderbolt cuts through Azumarill for a clean 2HKO.
Post-Game: Azumarill can solo AZ without much trouble, just be sensible in what you send it out against and Azumarill will do well in the Looker episodes and the final rival fight.
When freshly caught as an Azurill, the line will know Splash, Tail Whip, Water Gun and Water Sport. At level 7, your Pokémon will attempt to learn Bubble, which, despite the identical base powers between it and Water Gun, the potential speed drop is relatively useful, although its effectiveness is questionable given that the line is far from fast. Charm at level 10 does allow the line to improve its defense and is better than Tail Whip. Bubble Beam, which comes at level 13 is a solid early-mid game Water-move. However it should have been evolved by level 10 at latest to avoid missing out on a key asset if the Pokémon is intended to fight Viola. Beyond this, Helping Hand at level 16 is only useful in the uncommon double battles, Slam, which comes at level 20 is inferior to Return and Bounce at level 23, may have merit due to its coverage against Grass, Bug and Fighting-types, although it means missing out on Aqua Tail, which is terrible.
As a Marill, at level 10 Rollout provides a solid answer to Viola's Bug-types, while Bubble Beam and Helping Hand are learnt at the same time as Azurill would. The first key difference in their movepool is Aqua Tail, at level 20, or 21 as an Azumarill: this is a powerful STAB option that will likely serve your Pokémon well into the late game. Double Edge at level 25 is an extremely powerful Normal-type move, out-damaging full power Return, and Azumarill has the HP to weather the recoil better than most users. Use your own judgement between the two. At Level 31, Azumarill attempts to learn Aqua Ring. While this is the line's sole recovery move, it is outclassed by healing items. Next comes Rain Dance, at level 35, and does allow Azumarill to provide a degree of team support while buffing its STAB Water-moves. A move of significantly more interest, Superpower, comes along at level 42, and allows Azumarill to smash holes though Normal, Steel and Dark-type Pokémon, although the stat debuffs are unpleasant. Hydro Pump is Azumarill's strongest special STAB option, at level 46, and provides an interesting option for Thick Fat variants. Lastly, at level 50, Azumarill finally gets Fairy STAB through Play Rough, which is extremely powerful.
Through TMs, Azumarill gains a spectrum of options. Moves like Brick Break and Bulldoze provide additional coverage, and the former is a more consistent, although less powerful option for Superpower. Rock Smash serves as a solid early game option, but should be replaced by Power Up Punch, which has the same base power, but also an 100% chance to raise the user's attack by 1 stage. Return is always an option, as Water and Normal provides near unresisted coverage, and lacks the recoil of Double Edge. Waterfall is slightly less powerful than Aqua Tail, but lacks the miss chance. Surf, Scald, Ice Beam, Focus Blast and Grass Knot are all options for a special attacker, while Toxic and Light Screen are possible options for team support
Physical ***: Power Up Punch, Superpower, Play Rough, Waterfall/Aqua Tail
Grass-types: As always, Grass-type Pokémon work well with Water-types, covering Azumarill's Electric and Grass-type weaknesses. Stuff like Stun Spore and Sleep Powder are also valuable to let Azumarill have an easier time dealing with threats that its new allies would like to avoid. Preferably, get one that is not weak to Poison-types in order to avoid a shared weakness.
Fire-types: Adding to the traditional elemental core that is so common in many playthroughs, a fire-type, of which Kalos has plenty, provides an additional answer to Grass-types that threaten Azumarill and cover for its slightly problematic match up against Steel-types. Azumarill also devours opposing Rock, Ground and Water-types trying to extinguish the flames.
Steel-types: Azumarill has found itself with a new weakness to Poison-types in exchange for all the wonderful buffs given to it by the Fairy-type added to it. Thankfully Steel-type Pokémon are here to cover the aqua rabbit from its new enemy, atop of the old Grass-types that threaten it. It helps that Azumarill resists Fire and Fighting-type moves as well.
Fast attackers: Azumarill's main issue is that its low speed leaves it open to being picked off or 2HKO'd by some of the faster and stronger Pokémon in Kalos, despite its good bulk. However, said bulk allows it to be a pivot for frailer, faster attackers who can be the difference between life and death if getting the first hit in is vital.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Azurill Line Ratings
What Nature do I want? Azumarill is not making much use of its base 50 speed, so ideally you want Brave or Adamant on Huge Power ones and Modest/Jolly on those with Thick Fat, although defense boosting natures are also welcome.
Which Ability do I want? Huge Power by a mile. It's what makes the line a devastating physical tank in the first place and the power drop between Huge Power and Thick Fat is obvious from the get go.
At what point in the game should I be evolved? Azurill should be evolved before fighting Viola and before level 10 if you intend to use it in said fight. Otherwise it should be evolved as soon as possible, which given the long roads between gyms one and two, should not take long. Marill should be evolved at the first possible chance, the added bulk and power are crucial.
How good is the Azurill line in a Nuzlocke? It's post physical/special split Azumarill, it is going to be good. However, it cannot be understated how valuable this line is in Kalos. Its one of a kind typing grants it a phenomenal range of resistances to several common types and a powerful STAB combo. While those with Thick Fat may not be as destructive as their Huge Power counterparts, the line still is a incredible mixed tank, and proper team support only makes it harder for this pint sized powerhouse to be brought down.