Let's face it: we are about to see this thing everywhere, and for good reason: Aegislash is a force to be reckoned with.
So how do you counter an Aegislash?
I've seen it being discussed in a few other topics. I heard mention of using a Gliscor to heal and slam Mr. Aegis with repeated earthquakes. Earthquake seems to be the primary move, since it's non contact.
A smart Aegislash user will equip it with an Air Balloon. Lagging tail is another option to ensure shield form takes the enemy hit before striking.
Aegislash is steel type, so toxic stalling is off the table. However...
Willowisp: Burn that sucker, and cripple his attack stat so when he does try to strike, it won't hurt NEARLY as hard. Then finish him off.
"I can't speak for everyone else, but I'm angry because I'm not getting laid.." -stargazer64
Victini doesn't wanna be anywhere near a STAB 150 Base Attack Shadow Sneak, especially if it's Life Orb'd. Victini isn't going to want to V-Create into a King's Shield either.
Strategy for Aegislash: Predict the King's shield and set up appropriately with a Pokemon that's not weak to ghost. Aegislash's low base speed means it won't get the jump on you without Shadow Sneak.
For example, predict Aegislash's Shadow Sneak and switch in using something like say, Heliolisk. To be safe, you can attack this turn, or you can use some set up move. Aegislash risks a lot if it doesn't go into shield stance (because Heliolisk can outspeed and OHKO in Sword Mode), and because Heliolisk is a special attacker, it won't lose to King's Shield. If you're sure they'll activate King's Shield instead of attacking with Sacred Sword or something, use a set up move. If Heliolisk learns Electric Terrain, now would be a great time.
Heliolisk is my example because it resists and is immune to both of Aegislash's STABs despite being Sacred Sword weak.
"Religion keeps pressing B when we try to evolve." - pirategeorge" [3DS FC: 1934-0673-9848 (Pentao)]