Croagunk can be found in the Great Marsh as one of the Daily Binocular Pokemon, meaning they are not set to a specific area. In Platinum only, they can also be found on Route 212 - South as a standard wild encounter.
In addition, Toxicroak can also be found in the Great Marsh as a Daily Binocular Pokemon as well.
Gym #3 - Maylene (Veilstone City, Fighting-type): Even if you overleveled and evolved Aipom already, there's practically nothing you can do, since every one of Maylene's 'mons have a STAB Fighting move.
Gym #4 - Crasher Wake (Pastoria City, Water-type): It's pretty suicidal for Aipom, but Ambipom could try out this battle. Gyarados has Intimidate to reduce your best offensive stat, Dragon Rage to remove a good chunk of your health, Bite to inflict you some damage and cause you to flinch if it outspeeds, Brine whenever you have less than half of your health and Swagger to confuse you (and raise your attack as well, but its utility depends on how much you're going to hit it and not your face). A strong physical move like Return (try to get off a few Screeches/Tickles first), or even Thunderbolt, despite your poor special attack will work. If Swagger piles up, you could try going for a sweep, but remember you're still fragile. Quagsire is bulky on the physical side, has Slam to hit you hard with and Mud Bomb to try to mess with your accuracy; be also aware of Tail Whip, you don't want a defense drop. Return or Grass Knot will do the job. Depending on your level, nature and IV, Floatzel may or may not outspeed and hit you with one of its moves (while they're only medium power moves-except Brine when you're under half health- they're coming from a base 105 attack/ 85 special attack). Once again, Return is the key to defeat it.
Rival (Pastoria City): Again, Aipom should only face Starly, while Ambipom can try out against the others. Starly and Buizel can be dealt with Return; Ponyta is most likely 2HKO by Return; Roselia should fall to Return as well, but be careful of Magical Leaf coming from its special attack; Grotle is bulky and might KO you first; Monferno has both STAB and priority in the form of Mach Punch; Prinplup is doable, but be careful if it starts using Bide.
Gym #5 - Fantina (Hearthome City, Ghost-type): Because of type effectiveness, Ghost moves won't work on you, but your only offensive option are either Thief/Payback via TM or SE Astonish, resulting in a 60 base power move (90 if your ability is Technician). You could try to Baton Pass a couple of boosts when facing Gengar (STAB Poison Jab coming from its attack will 3-4HKO you). Another interesting (and funny) option is to equip an Iron Ball on Ambipom and then Fling it at one of the Ghosts, wrecking said Ghost in the process (a Flung Iron Ball results in a 130 base power Dark type attack), but once Flung, the item is consumed, so it's a one-trick-move.
Rival (Canalave City): The difficulty in this battle depends on your level and your stats. Staravia poses a problem with an attack drop due to Intimidate, Double Team shenanigans and STAB Take Down/Aerial Ace, so retaliate with Return or another strong move. Heracross is to be avoided due to Brick Break, Monferno as well because Mach Punch/Brick Break. Ponyta can be taken down with either Dig or Return, but be careful of its physical attacks (Stomp and Take Down). Roselia is physically frail, but watch out for its STAB Giga Drain if it survives. Grotle has the physical bulk to withstand 2 or even 3 hits, so watch out for STAB Razor Leaf's high crit chance and even Bite; you'll be outspeeding it, so nothing stops you from U-Turning on a better suited teammate while inflicting some damage. Buizel is another frail foe, so take it out with another string move, while Prinplup is doable, but its Bubblebeam stings.
Gym #6 - Byron (Canalave City, Steel-type): Ambipom can have its chance on this battle, provided with the adequate TM coverage. Despite possible troubles due to Hypnosis, Confuse Ray and Flash Cannon's possible special defense drop, Bronzor is doable, but its defenses will require at least a couple of hits; because it has Levitate, resort to either Brick Break, Shadow Claw and STAB Return (even if it's resisted) for strong damage (Technician users can consider Payback as well). Steelix should be avoided because of its very high defense, very high damage coming from Gyro Ball, Ice Fang and the possibility of it setting up Sandstorm; if you really want to face it (which isn't advised), go with Dig or Brick Break. Bastiodon has again high defense, but its typing makes it double weak to Brick Break and Dig; Flash Cannon and Ancient Power come off from a pitiful base 47 special attack but Iron Defense and Rest (plus an equipped Chesto Berry) makes it an enemy that will take potentially long to go down.
Saturn (Lake Valor): Kadabra is likely to be outsped, so hit it with Shadow Claw, Payback, Return or even U-Turn, if you're afraid of it surviving and retaliate with STAB Psychic; Bronzor is again a matter of a couple of strong hits, but Gyro Ball's damage and Rock Tomb's secondary effect might be annoying to deal with; Toxicroak is better left to another teammate, since it knows Revenge.
Mars (Lake Verity): Golbat main threat are poison from Toxic and confusion from Supersonic, so take it down with Return; Bronzong can be annoying because Gyro Ball has high damage and Iron Defense and Confuse Ray will likely prolong the fight (but you should know by now the drill if you want to defeat it with Ambipom); Purugly falls to Brick Break in one or two hits, but STAB Slash isn't merciful against you (even less if it crits), while Hypnosis and Fake Out will tick you off.
Gym #7 - Candice (Snowpoint City, Ice-type): Weather must be kept in consideration because of the Snover liners having Snow Warning as Ability. Snover can be taken out with Brick Break or U-Turn; Sneasel is wrecked by Brick Break; Medicham is definitely a no due to STAB SE Force Palm, despite it being weak to Shadow Claw; Abomasnow will tank your hits and Wood Hammer and Avalanche are both STAB and will most likely result in a kill, but you could U-Turn as a parting gift.
Cyrus (Galactic HQ): The most Murkrow can do is hit you with STAB Drill Peck, so hit it first with Return or, if you're afraid it surviving, U-Turn; Golbat can be dealt with easily, so Return once again; Sneasel is frail, be careful of its Ice Punch and Slash anyway, and hit it with either Return or U-Turn.
Saturn (Galactic HQ): Same verse as back in Lake Valor. Shadow Claw/U-Turn on Kadabra will deal with it, Bronzor now has Confuse Ray to be even more of a nuisance and Toxicroak has Brick Break, making it even more lethal than ever.
Mars and Jupiter (Spear Pillar, tag battle with rival): You have two ways to dealt with this double battle. The first consists in going in and fight normally, taking one side at time to minimize damage in case you're ganged on, just be ready to switch out against their aces, they're bulky and carry strong moves that might be lethal if they roll a critical hit. The second option is to set up Agility and/or Nasty Plot boosts, then Baton Pass them to a viable teammate able to accomplish a sweep with them. Snatch is also a move worth considering for this battle, as it will steal the Bronzor's Reflect and Light Screen, thus offering a bit of defensive support.
Cyrus (Spear Pillar): Leave Honchkrow to someone else: between high offensive stats and moves, it also carries Embargo (which will render impossible the use of bag and held items for 5 turns), thus making life harder for your speedy friend; Gyarados is to be avoided as well, too much power on its side and too little bulk on yours. Crobat is instead safer, but heal eventual poisoning and confusion; plus, if you're outsped, there's a chance Bite and Air Slash will cause some flinching shenanigans. Weavile is a gamble, as it carries strong STAB, coverage moves and a supereffective move in the form of Brick Break; if you're sure to outspeed it, then wreck it with a Brick Break of your own or leave a parting gift with U-Turn and allow a better suited teammate to deal with it. Overall, this isn't a favorable battle to take part in.
Dialga/Palkia (Spear Pillar): Alright, here's the plan: you aren't bulky enough to take the Dragons' STAB, so either throw a Master Ball, a Poke Toy or switch out.
Gym #8 - Volkner (Sunyshore City, Electric-type): This is another matchup that's not exactly advised to take part in. Raichu can cripple you with Thunder Wave, abuse RNG with Charge Beam's special attack's boosts and hit you supereffectively with Brick Break. If you don't have alternatives, one or two Digs should be enough to dispatch it. Octillery, despite its average bulk, is better avoided due to it having Sniper (which, for the record, makes critical hits inflict 3 times the normal damage instead of 2) and STAB Octazooka. Ambipom is a mirror match, but it can count on Nasty Plot to make its Technician-powered Shock Wave dangerous and Agility to outspeed you; plus, it can potentially Baton Pass away the boosts racked up to another opponent. Just take it out quickly. Luxray has Rivalry, but even in the case its ability is working against it, you shouldn't face it: Crunch and STAB Thunder Fang still hurt, Charge Beam can stack up multiple boosts on the long run and Thunder Wave will cripple your speed.
Rival (Pokémon League):
Elite Four Aaron (Pokémon League, Bug-type):
Elite Four Bertha (Pokémon League, Ground-type):
Elite Four Flint (Pokémon League, Fire-type):
Elite Four Lucian (Pokémon League, Psychic-type):
Champion Cynthia (Pokémon League):
Gym #4 - Maylene (Veilstone City, Fighting-type): It's likely you'll have Ambipom at this point of game, but other than U-Turning against Meditite/Machoke (depending on your IVs and nature, maybe even Lucario, but it's still a gamble) in order to bait a Fighting move on your switch, there's nothing else to do. Ambipom is not that bulky, so it won't withstand a STAB SE Fighting move.
Rival (Pastoria City):
Gym #5 - Crasher Wake (Pastoria City, Water-type): You should avoid this battle completely: Gyarados and Floatzel have high attack, making a crit a possible OHKO, while Quagsire is bulky and can cripple Ambipom via Yawn and Rock Tomb's Speed reduction.
Cyrus (Celestic Town):
Rival (Canalave City):
Gym #6 - Byron (Canalave City, Steel-type): Despite access to Dig/Brick Break's TM and Fire Punch via Move Tutor, everything has high Defense and STAB moves that hit hard: Thunderbolt/Flash Cannon for Magneton, Earthquake for Steelix, Stone Edge for Bastiodon. You could use Dig against Magneton, but let a bulkier teammate deal with the other two.
Saturn (Lake Valor):
Mars (Lake Verity):
Gym #7 - Candice (Snowpoint City, Ice-type): You can deal with Sneasel with Fire Punch/U-Turn/Brick Break, avoiding Piloswine and Abomasnow because bulk and STAB moves. Froslass is a decent matchup, as long as the field is devoid of Hail; be careful of Psychic's Special Defense drop and STAB Blizzard, then proceed to spam Fire Punch or Shadow Claw.
Cyrus (Galactic HQ):
Saturn (Galactic HQ):
Mars and Jupiter (Spear Pillar, tag battle with rival):
Cyrus (Distortion World):
Giratina (Distortion World):
Gym #8 - Volkner (Sunyshore City, Electric-type): Jolteon can be taken out with Dig, but you should switch in case Charge Beam's Special Attack's raises start piling up. The rest has high offensive stats, so don't even bother with them, but you could U-Turn as parting gift, maybe even baiting them in using a NVE move against your switch (just be careful of Raichu, it might outspeed depending on your Speed stat. You don't want Ambipom to eat a Focus Miss).
The lowest-leveled wild Croagunk will start with Poison Sting, Taunt, Pursuit, and Faint Attack. Poison Sting is pitiful as STAB, but you'll have to make the best of it. Taunt may not be that beneficial as most of the NPC Trainers won't rely on status moves, even if annoying. Faint Attack is better than Pursuit due to how little the NPC relies on switching, also it never misses. Revenge at Level 22 works if you know Croagunk is going to be attacked, and can survive it. It is generally a better choice than Rock Smash.