(Please note that the recommendations below are largely made for in-game play. A Mawile in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents.)
Long a fan favourite thanks to its interesting look, Mawile has nevertheless not been a great choice for Pokémon teams. Its stats are somewhat underwhelming, and prior to Generation VI it was a pure steel-type - not the greatest in the world. Fortunately, Generation VI has not only granted Mawile a dual typing, it has given Mawile a powerful new evolution. It's still not the best pokémon, but it's a lot better than it used to be.
Type: Steel / Fairy
Under normal circumstances, Mawile does not evolve. In Pokémon X and Y, however, you can obtain Mawilite which will evolve a Mawile into a Mega Mawile. As with all Mega evolutions, Mega Mawile only lasts until the end of the battle.
Bug, Dark, Fairy, Flying, Grass, Ice, Normal, Psychic, Rock. Immune to Poison and Dragon.
On its own, Mawile is not terribly impressive. It's largely a physical pokémon, eschewing both Special Attack and Defense for reasonably high Attack and Defense base stats. These don't help a whole lot, though, as Mawile can usually be pounded into the ground with even a remotely good fire attack, as its HP is pretty abysmal even at high levels. It's also one of the slower pokémon in the games, and generally loses out to its competitors.
Mega Mawile is another matter. Though its Speed is still poor and its HP low, it compensates by vastly improving its Special Defense. It's still possible to OHKO a Mawile, but it's much more difficult than before. Its Attack and Defense stats also shoot through the roof, granting Mawile substantially-improved sweeping capabilities.
Intimidate - Lowers the Attack of an enemy pokémon upon switching in. Overall relatively useful.
Hyper Cutter - Prevents Mawile's Attack stat from being lowered in battle. Fairly useful, especially if you're not working with a Mega Mawile, but I prefer Intimidate.
Sheer Force - Moves with a secondary effect receive a boost to their attack power at the expense of the secondary effect (flinching, burning, poisoning, etc.). This is a fantastic ability if you can find a Mawile with it attached, as it furthers Mawile's position as a slow, sturdy sweeper. Sheer Force is a hidden ability.
This depends entirely on the kind of Mawile you're looking to create. If you plan to exploit Mega Mawile, you're best off stacking Mawile with a variety of attacking moves. Iron Head and Play Rough make the best use of its STAB capabilities, Crunch isn't bad if you plan on sending Mawile up against psyshics or ghosts (or Sucker Punch if you're feeling lucky), Stone Edge can be handy against flying-types who aren't expecting trouble, and Return is all-around good once your Mawile is feeling friendly. Toss in Swords Dance and call it a day.
You can also still use Mawile in its more traditional role of a Baton Swapper, which is, again, made easier with a Mega Mawile's greater durability. Deploy Mawile against a pokémon that loves physical moves (preferably ones that fall into Mawile's copious list of resistances) and begin setting up with Double Team and Swords Dance. Attract is a good option if you need to stall while Mawile is prepping itself for the Baton Pass, assuming you get lucky on genders.
EV / Super Training
This depends entirely on what you plan to do with your Mawile. One that attacks should emphasize Mawile's Attack; one that stalls or Baton Passes is better off developing Defense, Special Defense, or both. Up to you. Regardless of the route you go, it's probably better not to emphasize Speed - at best Mawile will be middle-of-the-pack when it comes to speed, and then its other stats will suffer.
Catching a Mawile
Mawile is located in Glittering Cave in Pokémon X and Y. You'll have to spend time searching the narrow 3D maze to find one. Be prepared to look for a while, as they're one of the rarer pokémon encounters when you step on the darkened spots of the cave.
Got your Mawile? Now you'll want a Mega Mawile, which means finding Mawilite. After upgrading your Mega Ring, travel to Camphrier Town and check out the castle in the north. Assuming you visit between 8 and 9 PM, you'll find Mawilite sparkling away on the second floor.