Overview
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While Aegislash has a good defensive typing and an ability that allows it to switch between great bulk and power, it has several flaws that prevent it from doing its job consistently. Its signature move, King's Shield, lets it switch forms and check physical attackers like Zekrom, but allows opponents a free turn to switch or status it. Aegislash's unique typing and bulk make it the best counter to Geomancy Xerneas, but it lacks recovery outside of Leftovers and is weak to some other common attacking types in the Ubers metagame, meaning it can be worn down easily. One key advantage Aegislash has over other walls is that it cannot be trapped by Mega Gengar, and actually OHKOes it with uninvested Shadow Ball 81.3% of the time while living one from Gengar. Unfortunately, it lacks strong STABs and Speed, limiting it to a defensive role. Aegislash is a Pokemon that is useful because it is the best counter to Geomancy Xerneas, and nothing beyond it besides occasionally spreading Toxic, and it should thus never be the first choice for a team.
Specially Defensive
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name: Specially Defensive
move 1: King's Shield
move 2: Toxic
move 3: Gyro Ball / Iron Head
move 4: Shadow Ball / Pursuit / Shadow Sneak
ability: Stance Change
item: Leftovers
evs: 252 HP / 4 Atk / 252 SpD
ivs: 0 Spe
nature: Sassy
Moves
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King's Shield allows Aegislash to scout, get more Leftovers recovery, and most importantly, change back to its Shield form. It also allows it to lower the physical attack of anything that hits it with a contact move, most importantly during Outrage. Toxic allows Aegislash to beat walls like such as support Arceus and Lugia and synergizes great with King's Shield. It also lets you Aegislash beat Substitute Ho-Oh if you it hits it on the switch. Head Smash also beats Ho-Oh on the switch and does a ton lot of damage ('a ton' sounds informal; it's advisable to maintain a formal tone in an analysis) to Yveltal and Shaymin but means you Aegislash loses the ability to Toxic any wall, meaning Toxic is usually the making Toxic a better option overall. Gyro Ball easily OHKOes a boosted Xerneas and can do chip damage to many faster Pokemon switching in, like Darkrai. Iron Head is much more reliable but can never OHKO Xerneas even after Stealth Rock, though is much better against defensive Xerneas, which always 3HKOs it, while Gyro Ball barely 4HKOs though it always 3HKOes defensive Xerneas, which is barely 4HKOed by Gyro Ball. Shadow Ball hits some frailer Pokemon hard, but is too weak uninvested to do any major damage to bulkier Pokemon, failing even to take out a Substitute from Ho-Oh; (semi-colon) but do note, however, that it does a lot of damage to Mewtwo, specifically Bulk Up Mega Mewtwo X, which Aegislash can live any hit from when near full health. It also 3HKOes Groudon and 2HKOes Landorus-T and Hippowdon, meaning they must be careful while switching into it. Pursuit allows Aegislash to damage Mega Gengar as it switches, since it can not trap or OHKO Aegislash most of the time, even with Shadow Ball. Shadow Sneak can be used over Shadow Ball to revenge kill very weakened sweepers like Arceus-Ghost, but will not do any major damage otherwise. It does limit leads like Deoxys-A to a single layer of hazards, however.
Set Details
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Leftovers synergizes well with King's Shield and is Aegislash's only form of recovery. The EVs and Sassy nature give Aegislash maximum special bulk without compromising either attacking stat; (semi-colon) and they should be used even without Shadow Ball to increase the power of Gyro Ball and lower the its chances of being hit in Blade form. Running max HP and Speed with a Jolly nature and Head Smash destroys any Ho-Oh with under 100 EVs, acting as an amazing lure, but this causes a ton lot of recoil and a lot less significantly reduces bulk, as well as forcing you to run Iron Head over Gyro Ball and Shadow Sneak over Shadow Ball.
Usage Tips
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Stance Change gives a well-played Aegislash great bulk and power, but defensive Aegislash should not be taking unnecessary hits in either form due to lack of recovery. Aegislash is not going to wall everything and should not be the only defensive Pokemon on your team, so do not keep it in against something that can overpower it, (comma) like such as Kyogre. If you can look at the opponent's team and see Aegislash is useless, (comma) then feel free to let it die, but using King's Shield and dropping the opponent's Attack the turn before dying can give a teammate a chance to set up as they are forced to switch. Your Aegislash's STABs are weak without investment, so stick to spreading Toxic until you have to attack, because you don't want Aegislash to be stuck in Blade form; outside of Pokemon like Gengar or Xerneas, they it will mostly be doing only chip damage. Switching when in Blade form allows you Aegislash to come back in Shield form. Attacking then switching allows you Aegislash to get chip damage, then switch out yourself without needing to use King's Shield. Play conservatively and avoid a burn, especially if you see a Xerneas, until you can scout what it's using and play around it. If it is running Substitute, you must be very careful about using King's Shield, because then Xerneas, while weakened and in range of revenge killing, will beat Aegislash 1-on-1.
Team Options
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Defensive Ho-Oh resists Fire, (add comma) and is immune to Ground, and has good Special Defense to take hits from Yveltal, and Aegislash can take Rock-type moves for Ho-Oh, but neither can handle Kyogre. Landorus-T is immune to Earthquake, beats many common Fire-types, and can also set up Stealth Rock. A defensive Yveltal can beat Excadrill and Arceus-Ghost. Arceus-Rock beats Ho-Oh but also loses to Yveltal that have Focus Blast. Palkia can beat Kyogre, and Aegislash can take Dragon-type attacks for Palkia. Do not build around Aegislash, though; it is a utility Pokemon before anything else, (remove comma) and should only be added to a team that needs it.
Other Options
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Aegislash has the stats to go offensive, but not the movepool or Speed, and this is better left to Arceus-Ghost as it can boost and recover, (comma) and has the Speed to sweep. Using Flash Cannon as a Steel STAB would mean boosted Xerneas can wall and destroy you Aegislash, and Shadow Ball hits everything Flash Cannon would. Sacred Sword can be useful to hit Ferrothorn, Arceus-Steel, or Arceus-Dark, but is very weak uninvested, and the other moves are much more important. You can try using a physically defensive set, but this means you are Aegislash is always 2HKOed by Hidden Power Fire from boosted Xerneas, (remove comma) and loses to a Shadow Ball from Mega Gengar. Boosting sets, (remove comma) such as Swords Dance or Autotomize should never be used, as Aegislash has coverage issues and weak moves.
Checks & Counters
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**Yveltal** - Yveltal OHKOes Aegislash with Dark Pulse, (remove comma) and can set up a Substitute if it predictes King's Shield. Toxic becomes a wasted turn if Yveltal Substitutes, and only the rare Head Smash can scare it.
**Ho-Oh** - Ho-Oh also sets up a Substitute on Toxic and can do a lot of damage to Aegislash while possibly burning Aegislash, it and not activating King's Shield. It only fears Head Smash.
**Groudon** - Groudon takes some damage from uninvested Shadow Ball, (remove comma) but can always switch in to it and still KO Aegislash with Earthquake before dying. It hates Toxic, but needs to be hit on the switch with it.
**Kyogre** - Aegislash does not have the bulk to take Water Spouts, (add comma) and defensive ones Kyogre can Rest if Toxic damage gets too high. Aegislash also fears Scald burns.
**Darkrai** - Darkrai takes decent damage from Gyro Ball, but Substitute variants set up over it on Aegislash and then can then put it to sleep or just kill it off with Dark Pulse.
**Arceus-Dark** - Arceus-Dark easily takes hits from Aegislash while getting rid of Toxic with Refresh or avoiding it with Substitute, and then can beat Aegislash with a Judgment. (period)
**Arceus-Normal** - Arceus-Normal hates Toxic, but it can use Earthquake to get around King's Shield, (remove comma) or set up on King's Shield. Aegislash can beat unboosted ones without Earthquake, but those are rare. Arceus-Normal occasionally uses Lum Berry as well.
**Fire-types** - Aegislash requires prediction to beat Mega Charizard X and Blaziken. It loses to Mega Charizard Y, Heatran, Arceus-Fire, and Reshiram all the time, though.
**Dialga** - Dialga is immune to Toxic and doesn't take much from anything Aegislash can use against it; it and can beat Aegislash with Fire Blast
**Arceus-Steel** - Immune to Toxic and can set up on it or Burn it.
**Arceus-Poison** - Immune to Toxic and can set up on it or Burn it.
**Toxic-immune Arceus formes** - Arceus-Steel and Arceus-Poison are immune to Toxic; can set up on Aegislash or burn it.
**Excadrill** - Excadrill is immune to Toxic and can Earthquake it Aegislash.
**Gliscor** - Like Excadrill, Gliscor is immune to Toxic and can Earthquake it Aegislash, but it takes a decent amount of damage from Shadow Ball.