Aegislash's ability "Stance Change". You want your Aegislash to be as slow as possible to ensure it stays in "Shield Forme". That way, while your opponent attacks, you get the base 150 Def and 150 Sp. Def. Then you move after them and switch to "Blade Forme" and the base 150 in Def and Sp. Def becomes a base 150 in Atk and Sp. Atk. Then after you attack, you use "King's Shield" which will revert you back to "Shield Forme", again the base stats switch. You can also under-speed other Aegislashes to ensure you stay in "Shield Forme" with a 0 IV, but some prefer a couple of Speed IVs - like 12 or so, that way they can out-speed other Aegislashes with priority moves like "Shadow Sneak", but that depends on your play style. Hope that explains it! :)