Extremekiller Arceus with Earthquake (i.e. the most common third move) absolutely slaughters this set once the balloon is popped, for the record. Also it's way too prediction reliant even if the Arceus doesn't have Earthquake because they can just predict the King's Shield and SD again.I have been using Aegislash for Smogon Ubers beta and its doing a lot of work at the moment.
Aegislash @ Air Balloon
Ability: Stance Change
EVs: 252 Atk / 252 HP / 4 SDef
Brave Nature
IVs: 0 Spd
- King's Shield
- Shadow Sneak
- Gyro Ball
- Sacred Sword
King's Shield absolutely puts a stop to Extreme killer Arcues, the only common move Arcues runs that can hit him is shadow claw and I can just king shield that and cripple Arcues. Sacred Sword when they try to SD up or attack me(after the attack drop).
Gryo Ball absolutely wrecks Xerneas, Aegislash walls Xerneas hard and 1 gyroball after a Geomacy will kill him. Shadow Sneak wrecks Mewtwo and works as a excellent revenge killing move.
His speed catagory gives him a big advantage, since most threats will outspeed him, Aegislash can very well use both his immense bulk and immense attack power in the same turn, while still outspeeding pokemon like Ferrothorn and hitting him with a sacred sword.
I put 4 evs in special defense so Genesect won't get the SPA boost and hit it with a flamethrower.
Apart from that its still a very solid set, though, since Aegislash does have great typing as well as the ability to counter Psyshock/Focus Blast/Moon Blast Xerneas like you said. (Is 2HKO'd by +2 Thunder(bolt) though)