I think it's better for Aegislash to attack after it's been attacked. I've seen some people try to go for a Swords Dance -> Shadow Sneak strategy with him that, while very nice, can result in someone surviving the attack and hitting it with a super-effective move that will OHKO him because he's still in Blade Mode. So, what I like to do is spend a few EVs on both defenses OR 252HP/252Attack. Endure an attack in Shield Mode and THEN use either Shadow Claw, Iron Head or Sacred Sword. Even if it doesn't kill, the attack will hurt a LOT and he can immediately use King's Shield to block whatever comes at him, including the chance to lower two Attack stages if it blocks a physical move. The same strategy pretty much works for a Special Aegislash, just switch the 252 EVs to SpA and use Shadow Ball and Flash Cannon.
This pokémon is beautiful, definitely one of the best out there.
This pokémon is beautiful, definitely one of the best out there.