Archen can be revived from a Plume Fossil in Nacrene City. The Plume Fossil is found in the Relic Castle.
Archen is one of the best jack-of-all-trades of the whole fifth generation. Its line is the very definition of a glass cannon: five weaknesses, the fact that those weaknesses are to fairly common types to boot, and fairly low defenses to back them up... however, so long as Archeops is never hit by a moderately powerful (or stronger) opponent, it will never encounter a single problem on its ventures. Its movepool is amazing on the physical side and workable on the special side, making it capable of filling numerous offensive roles. A great addition to any team.
N (Nimbasa City): Unless Archen is already at Acrobatics level, this fight will be somewhat difficult. N's Pokémon are not weak at all; they will put a serious dent in the poor fossil's frail defenses. If Archen does have Acrobatics, Scraggy will be a piece of cake, and Darumaka should also go down easily to AncientPower, since Archen resist all of its moves; Archen cannot OHKO it unless it has a few levels of advantage, but the enemy will not come close to OHKOing Archen, ever. Sandile can be 2HKOed with Pluck at worst, providing that at least one Pluck comes from before Archen enters Defeatist range. Note that Scraggy may not be OHKOed without Acrobatics, unless Archen is above level 25, in which case Pluck should be sufficient, barring a much lower than average Attack. Sigilyph is absolutely horrifying for Archen: it has high defenses and will definitely live a hit from it, and can fire back with a STAB Psybeam that will almost certainly be a OHKO if it scores a critical hit.
Gym #4 - Elesa (Nimbasa City, Electric-type): Do not try this with Archen. All of her Pokémon have Volt Switch, and the frail fossil will never be able to tank one hit. The two Emolga might outspeed it, too, and AncientPower - Archen's best Rock move up to this point - is definitely not powerful enough to one-shot them either way.
Cheren (Route 5): Leading with Archen is possible, however a little dicey due to the damage it will likely take from its enemies. Liepard's Pursuit will not deal a lot of damage if Archen stays in, and Acrobatics will score a clean OHKO unless Archen's Attack is below average. Tranquill will be easy to handle, as will be Pansage or Servine. Acrobatics will take down Pansear too, but do not risk it against Panpour unless Archen is still at full health or around there, since it knows Scald; unless its nature is detrimental to Attack and its IV is pitiful, though, this will not be an issue. Archen can laugh at Pignite, but should stay away from Dewott no matter what.
Gym #5 - Clay (Driftveil City, Ground-type): Archen is not a good Pokémon to use here. Krokorok's Intimidate prevents it from OHKOing with Acrobatics, which would otherwise be possible, and its Crunch is a 2HKO, which means Archen will be brought down to Defeatist range. Archen can defeat Krokorok if there are no better options, but doing so will cost Set players a turn to switch out, as well as exposing Archen to lethal critical hits. Palpitoad and Excadrill are impossible either way, due to Muddy Water and Rock Slide respectively.
Bianca (Driftveil City): For the starters and the elemental monkeys, the matchup has not changed: anything Grass is easy for Archen to beat and Fire is an OK matchup, but do not touch anything Water with the exception of Panpour, which can be OHKOed by any Archen with a decent enough Attack stat. Herdier and Musharna can both be 2HKOed with Acrobatics, but Archen can only fight one of them, as it will be brought to Defeatist range if it attempts both.
N (Chargestone Cave): Every single one of his Pokémon has STAB that is super effective against Archen. Going into this fight is basically asking for brutal murder. Be sensible: unless Archen needs to KO Joltik with Acrobatics, which is once again possible but only for Archen with higher than average Attack, then the fossil should not participate in this battle.
Gym #6 - Skyla (Mistralton City, Flying-type): Archen has potential to truly shine against this gym. If it gets to Crunch level, it can take down Swoobat very easily without forgoing the Eviolite, which otherwise nerfs Acrobatics; Unfezant will eventually fall as well, since Archen resist both of its STABs and can wear its health down with AncientPower, an average 2HKO, and the fossil also resists all of Unfezant's moves. Leave the Swanna alone, however, as it both outspeeds and 2HKOs with BubbleBeam, even with the Eviolite. This is not an issue for Archeops, which can score a near-OHKO with Acrobatics; BubbleBeam is still a 2HKO, though, so critical hits will end even an already evolved Archeops.
Cheren (Route 7): Acrobatics is a recipe for victory in this fight. Archen must be evolved first, otherwise Archeops will not be taking on the elemental monkeys so confidently. Dewott should still be avoided, as the Acrobatics OHKO is very unlikely; Simipour can be OHKOed more easily, but running calculations on Archeops' actual stats is advised first, as a critical Scald will kill it.
Gym #7 - Brycen (Icirrus City, Ice-type): Vanillish and Cryogonal will both go down easily to any physical Rock move (preferably Rock Slide), or even just Acrobatics, which may actually be better due to its perfect accuracy. However, keep Archeops away from Beartic, as its potent bulk will leave it with at least a sliver of health no matter what move Archeops uses (short of Stone Edge, which is still a huge risk due to the low accuracy) and then kill Archeops with Icicle Crash. Recall that, despite being a Rock-type, Archeops is still weak to Ice moves.
Bianca (Route 8): Same matchup as the previous battles: keep away from all things Water; Stoutland in itself is not dangerous, but its Intimidate reduces Acrobatics from a 2HKO to a 3HKO, lengthening the fight. Samurott's Aqua Jet is also not horrible, but its priority will bring Archeops down to Defeatist range before it can damage Samurott, and that is just not going to work. If Archeops has nullified the effects of Intimidate after Stoutland, Simipour can be OHKOed with Acrobatics - once again, Archeops will need to have a good Attack stat to actually pull this off - and Musharna can be 2HKOed before it deals much damage.
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type): Proceed with caution, the leader's Pokémon are fairly bulky and somewhat dangerous. Fraxure can be OHKOed in most cases, but Druddigon is only a 2HKO; its Revenge could also bring Archeops down to Defeatist range, and if it does, the 2HKO will no longer happen. However, since Revenge boosts its power by having the user take damage first, Archeops can heal during the battle to stay within the healthy range and then unleash a full-power Acrobatics again, thus defeating Druddigon too. Lastly, Haxorus will most likely lead with Dragon Dance, outspeeding Archeops on the second turn. While Acrobatics is a 2HKO normally, it will no longer be after the ace attacks, unless it somehow goes for Dragon Tail (which has negative priority) over Assurance. Winning is possible, but if a prediction fails, Archeops will eventually need to bail out, and needs a supporting member that can pick up the fight from there.
Cheren (Route 10): Archeops will be fine handling anything its STAB moves are super effective against, but do not try it with Water-types. Archeops may calculate Acrobatics damage against Simipour, which it is somewhat likely to OHKO, but that depends on Archeops' Attack stat and either way, Samurott is not an appropriate target.
Elite Four Shauntal (Pokémon League, Ghost-type): Archeops can 2HKO Cofagrigus or Golurk with Acrobatics, but if it attempts to take on both of them, it will need to avoid critical hits and be healed: the second one that hits will activate its Defeatist, though otherwise, even their best moves can do no more than 50% damage. Chandelure dies to Rock Slide. Do not fight Jellicent; Archeops can live a Scald unless it is a critical hit, but it will not be healthy enough to KO afterwards.
Elite Four Marshal (Pokémon League, Fighting-type): Everything in Marshall's team pales in front of Archeops' Acrobatics. However, do not go against his Sawk when it is at full health, because it has both Sturdy and Stone Edge, and it will brutally murder Archeops. Instead, Archeops can use U-turn on Sawk to break its Sturdy and bring in a Pokémon that is not wrecked by Rock attacks, potentially letting Archeops back in later on. Running calculations before going in is still a wise idea; the OHKOs may not be possible for Archeops with detrimental natures and/or poor Attack IVs.
Elite Four Grimsley (Pokémon League, Dark-type): Scrafty, an otherwise potentially serious threat for most teams, is completely ridiculed by STAB Acrobatics. Liepard is not threatening either. Krookodile has Intimidate, though, and will never go down in one hit; it is better to switch or use U-turn against it. Bisharp has Steel STAB with a deceptively high damage output, and Archeops just cannot stand that much power, especially as its main STABs are both resisted; switch out, or U-turn again, against Grimsley's ace.
Elite Four Caitlin (Pokémon League, Psychic-type): All of her Pokémon except Musharna are fairly dangerous, carrying either an Electric or an Ice move; Musharna "only" has Charge Beam, which is less scary than STAB Psychic. Archeops can 2HKO Sigilyph, Musharna and Gothitelle with a combination of Acrobatics and Crunch, or just two Crunches, but only Musharna cannot kill Archeops even with a critical hit; the other two both have moves that will take Archeops below half health, which is not an issue with Crunch, but does mean that it will never be able to defeat more than one Pokémon Caitlin has. Pick Archeops' fights according to which Pokémon the rest of the team is least likely to handle; Archeops can provide support, but it cannot go solo.
Reshiram/Zekrom (N's Castle): Archeops' Rock Slide backed by a Rock Gem has a high chance of OHKOing Reshiram. In the event of a miss or a non-KO, Reshiram is still highly unlikely to kill Archeops with a critical DragonBreath, so Archeops will maintain the advantage unless two misses occur in succession. Zekrom should not be attempted at all, as a single Fusion Bolt will result in Archeops' death.
N (N's Castle): Any attempt to take on Carracosta, Vanilluxe and Klinklang will end in failure on Archeops' end, so avoid that. Archeops may have super effective moves, but they have plenty of bulk, and Carracosta's ability is Sturdy. Vanilluxe goes down to Stone Edge or a Rock Gem-boosted Rock Slide, but nothing less, and both of those moves have less than perfect accuracy. Archeops can mirror Ghetsis' own Archeops fine if it can outspeed it, for example by having a Speed-boosting nature or being higher levelled. Zoroark, once unmasked, should not be a problem. Archeops may attempt to beat Reshiram only if it is still holding a Rock Gem, because it will need that boost. Absolutely stay away from Zekrom.
Ghetsis (N's Castle): Due to Ghetsis' Pokémon's coverage and bulk, it is much better for Archeops to just plain stay out of this fight. Archeops can at best 2HKO Cofagrigus, Bouffalant or Seismitoad; all of their hits do more than 50% to Archeops, but cannot kill unless they score a critical, whereas Bisharp and Eelektross are out there to destroy the primal bird. Hydreigon comes close to killing with Surf, but does not manage just barely; the issue is that Archeops cannot KO it soon enough, either, with Defeatist coming into play. U-turn is the best option against it, and will help a proper counter defeat it, since Hydreigon is also weak to Bug.
Post-Game: Threatening as it might be, whatever cannot stand a STAB Acrobatics at full power will stand no chances against Archeops. It will also get access to more coverage after the credits roll, notably Earthquake, which is a godsend for such a versatile Pokémon.
Archen's initial movepool consists of Pluck, AncientPower, Agility and Quick Guard. Pluck and AncientPower are good enough on their own at this level, but nothing can compare with the amazingness of Acrobatics, learned at level 28. After getting this move, never give Archen a held item again (with the only exception being the Flying Gem, which will boost the move to an astounding base power of 165 before STAB). Acrobatics is amazingly strong, with effective base 110 power and STAB, plus 100% accuracy; it will take down almost every enemy that does not resist it and does not have absurd Defense. More coverage comes at level 31 in the form of DragonBreath, which is not bad but probably will not be really needed throughout the journey, due to STAB Acrobatics being more powerful than super effective DragonBreath. At level 35 it gets Crunch, which sounds really good, but an itemless STAB neutral Acrobatics will outdamage it against anything that does not resist Acrobatics anyway. After evolving, it learns Endeavor at level 40, which is a move Archeops will never be using, and U-turn at level 45, which is still outclassed by Crunch and Acrobatics (both in base power and in type effectiveness), but may come in handy often especially for Set mode players, who will need to switch their Archeops out of the battlefield a lot. Being able to switch out while doing damage is very useful, especially coming off Archeops' amazing Attack, even when in Defeatist range; it can also help break some enemies' Sturdy. In addition, having U-turn alone makes Archeops a fantastic team lead. At level 45, the long awaited physical Rock STAB comes in the form of Rock Slide; Archeops will probably never let go of this move, unless it eventually goes for the less accurate but more powerful Stone Edge, which is not advisable: a miss is likely going to mean the end for an Archeops. Dragon Claw, learned at level 56, may prove useful against N's legendary dragon, but it is probably best to not take that gamble, especially as an itemless STAB Acrobatics does more damage to Reshiram and Zekrom will beat Archeops anyway. Lastly, at level 61 comes Thrash, which is the apotheosis of uselessness on a Pokémon with such great coverage.
The TM department has much to offer to the Archen line. Archeops can opt for Stone Edge over Rock Slide if power is preferred to accuracy, but Archeops will probably need accuracy more than power if it ever needs to go for Rock moves over Acrobatics. Earthquake is another all-around solid choice and popular option, but fast Electric-types might still outspeed Archeops and the move is sadly not available until after the credits roll. Shadow Claw is a nice filler move to have while Archeops waits for Crunch, as its type effectiveness is basically the same and its power is not inferior by far. Bulldoze and Dig are fairly reliable in the early- and mid-game, but even when they are super effective, neutral STAB Acrobatics will deal more damage. While level grinding, Fly can be useful if Archeops is holding a Lucky Egg as that lowers the damage output of Acrobatics, and provides utility. Itemless Acrobatics and Rock Slide do so much damage to anything that Archeops might even want to keep Fly as a filler move.
If Archeops happens to have poor Attack and/or good Special Attack, it may opt for some of its available special moves - mostly Focus Blast, though Archeops can consider DragonBreath as well, if the team is pressed for special Dragon coverage. Sadly, AncientPower is all the special STAB Archeops will ever get unless it gains access to a high base power Hidden Power of an interesting type. Do not waste Archeops' time with stat-boosting moves or Sandstorm, Archeops is too frail to play any role other than the resident glass cannon.
Recommended moveset: Acrobatics, Rock Slide, U-turn, Dragon Claw / Crunch
Fighting-types: The type that has best synergy with Archeops is Fighting; it provides good coverage for over half its weaknesses, and Archeops can usually cover Fighting weaknesses well enough, despite not being able to offer a resistance to Psychic moves. Fighting-types are also fairly common in the game.
Grass-types: Unlike Electric-types, Grass-types share an Ice weakness with Archeops, but they are better than Electric-types at functioning as general counters for both Water and Electric Pokémon since they sport a Water resistance. Grass-types are also much easier to find than good Electric-types.
Physical tanks: Taking hits is not Archeops' cup of tea, and its Defeatist ability will make it need many more switch-outs than the average glass cannon; in addition, its U-turn can only be proficiently used if there are good switch-ins on the team. A physical tank or wall must thus be present.
Special tanks: Archeops' defenses are equal, and it needs special coverage as much as it does physical coverage. Several special walls are available in Unova, and at least one should accompany Archeops on the journey; this is relevant especially in the Elite Four stretch, against Shauntal and Caitlin.
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.
Archen Line Ratings
What Nature do I want? Adamant is obviously the best due to Archeops's great physical movepool, although any Attack-boosting nature in general will work. Special Attack-boosting ones can also be decent, as Archeops' special movepool is not bad either.
At what point in the game should I be evolved? As soon as possible. Archen is much too frail to be able to stand a hit, and Archeops does not really have grand bulk, either; in addition, having Archen hold the Eviolite hinders its performance in battle, halving the base power of Acrobatics. The evolution should preferably occur around Mistralton City, whether before or shortly after Skyla.
How good is the Archen line in a Nuzlocke? Amazing in the physical offensive department. Despite its frailty and hindering ability, it can learn almost all of the most powerful physical moves available in the game, as well as having a respectable special movepool and a high base Special Attack stat on top of that. Simply put, one can never go wrong with Archeops, and itemless Acrobatics will dish out endless pain to almost anything. Just do not try using it as a wall, and if its health drops below half, make sure to heal or switch out.
Weaknesses: Rock, Steel, Water, Electric, Ice
Resistances: Normal, Flying, Poison, Bug, Fire
Neutralities: Fighting, Ghost, Grass, Psychic, Dragon, Dark