Pokémon X and Y Walkthrough, Pokémon Move Sets: Aegislash
(Please note that the recommendations below are largely made for in-game play. An Aegislash in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If you have your own move set you'd like to share, feel free to toss it into the comments.)
A ghostly creature that appears to inhabit ancient arms, Aegislash is one of the stranger pokémon to make an appearance in Generation VI. It's capable ot swapping between two different formes in battle, and though the resulting changes can be a little strange to adapt to at first, they ultimately result in one of the most potent fighters of the new generation.
Type: Steel / Ghost
Aegislash begins life as Honedge. Once Honedge reaches level 35, it evolves into a Doublade. To turn Doublade into an Aegislash, you'll have to track down a Dusk Stone. You can either proceed through Super Training until you earn a Dusk Stone from the Aegislash challenge, or you can speak to the Team Flare Grunt in Laverre City after you've beaten the game.
Bug, Dragon, Fairy, Flying, Grass, Ice, Psychic, Rock, Steel. Immune to Fighting, Normal, and Poison.
Dark, Fire, Ghost, Ground.
Aegislash is easily one of the most innovative pokémon around as far as its stats go. When in its Shield Forme, which is how it begins every battle, Aegislash is heavily weighted towards Defense and Special Defense. Its offenses are very low. Once it swaps to Blade Forme, though, Defense and Attack swap, as do Special Defense and Special Attack. Herein lies Aegislash's strength, as you can concentrate on only two of its stats but still essentially cover all four. That said, its HP and Speed will always be rather low.
Stance Change - When Aegislash uses a move that causes damage, it changes from Shield Forme to Blade Forme, swapping its offensive and defensive stats in the process. Using King's Shield switches Aegislash back to Shield Forme. No choice in abilities, really, and you wouldn't want to lose Stance Change anyway. (Though getting rid of it in battle is a great way to cripple am Aegislash.)
Aegislash has the potential to be one of the best sweepers on your team with the proper setup. Though its HP is not terribly high, Aegislash can easily withstand most attacks thanks to its titanic defenses. Start your battles with moves that will raise its stats, primarily Autotomize to raise its Speed and Swords Dance to raise its Attack. Once that's done, use an offensive move to swap to Blade Forme an annihilate your opponents. My personal favourites are Shadow Claw for STAB damage and Return for almost everything else, but you have other options as well. I recommend the naturally-learned Sacred Sword, though you'll have to wait for Doublade to reach level 51before you can learn it.
This begs the big question: Should you bother with King's Shield? That depends on how you wind up developing your Aegislash, though I don't recommend it. Make your Aegislash fast enough with Automotize before switching formes and you won't usually need to switch back. Better to have more variety with extra attacks.
EV / Super Training
Defense, Special Defense. That's all you need. Aegislash's defenses are usually strong enough to outlast a single attack, and since your stats swap when changing formes, there's no point pumping EVs into offense and defense alike. HP is a possibility, as well, though don't overdo it.
Catching an Aegislash
To get your Aegislash you'll have to develop a Honedge to level 35, then evolve the resulting Doublade using a Dusk Stone. Honedges only appear in one place, on Route 6, though they're common enough that you'll probably run across one on your first trip through the area.