Moves
Play Rough is Mega Mawile's devastating STAB move, smashing through anything that doesn't resist it. Sucker Punch is Mega Mawile's powerful priority move, allowing it to pick off weakened foes and clean offensive teams later in the game. It also avoids Aegislash's King's Shield, a great utility for a physical attacker. Swords Dance boosts Mega Mawile's Attack to absolutely apocalyptic levels. It might seem like overkill to boost what's already the highest Attack stat in the game, but Mega Mawile greatly benefits from Swords Dance for two reasons. Mega Mawile might be strong, but it requires a boost to break down walls such as Cresselia and Porygon2 effectively. The other reason is that an Attack boost makes sweeping through weakened offensive teams with Sucker Punch a much more realistic possibility.
Knock Off is usually the easiest move to fit in the last slot. While its coverage might be redundant with Sucker Punch, it avoids the mind games Sucker Punch can be locked into, and its item-removing power can be extremely useful against certain switch-ins. Knock Off also allows Mega Mawile to wallbreak more easily, as it removes Porygon2's Eviolite and OHKOes Cresselia after a boost. Fire Fang is the more specialized option, having a shot at OHKOing Ferrothorn and 2HKOing opposing Mega Mawile even after Intimidate. Fire Fang also has a shot at OHKOing Skarmory after Stealth Rock and a Swords Dance boost.
If you wish, there are even more specialized tools for Mega Mawile if you feel Knock Off and Fire Fang are not helping your team much. Facade can be used to laugh in the face of Will-O-Wisp, giving Mega Mawile an attack even more powerful than its STAB move when statused! Rock Slide and Stone Edge are acceptable attacks for crushing Talonflame and Charizard on the switch, and Rock Slide in particular can cause paralyzed foes to flinch. If you simply want a reliable, accurate STAB move, Iron Head can be used, with the bonus of being able to cause paralyzed targets to flinch, though using it comes at the cost of coverage. Finally, if Sucker Punch's mind games are simply too much for you to handle, you can use Substitute in the last slot at the cost of coverage.
Set Details
Mawilite is the entire point of Mawile, so holding it is essential! Maximum HP investment along with Intimidate gives Mega Mawile the survivability it needs to launch off attacks, while maximum Attack investment and an Adamant nature ensure that Mega Mawile's KOing power is through the roof! If you are using Mega Mawile in Trick Room, consider a Brave nature and 0 Speed IVs to underspeed Quiet Aegislash and opposing Mega Mawile. If you want a bit of extra bulk, consider an EV spread of 252 HP / 212 Atk / 44 Def. This EV spread allows a full-health Mega Mawile to survive Choice Band Talonflame's Flare Blitz after an Intimidate, which can ruin your opponent's win condition in a pinch!
Usage Tips
It's tempting use Swords Dance right away, but resist the urge earlier in the game. Mega Mawile might be powerful, but Sucker Punch doesn't OHKO everything with a boost, and Mega Mawile is also extremely vulnerable to Substitute and Will-O-Wisp. When your chances of sweeping aren't clear, Mega Mawile is better off using Play Rough or Knock Off, because even without a boost, these moves deal a tremendous amount of damage. Swords Dance is best used when you need Mega Mawile to break a defensive Pokemon or know that Mega Mawile will be able to sweep the rest of the opposing team.
Mega Mawile might have decent bulk and a great ability and typing, but switching it into neutral attacks can ruin chances to launch attacks or set up. Try to goad your opponent into bringing in Pokemon that struggle against Mega Mawile. For example, double switching your physical threats from a predicted switch to Porygon2 or Cresselia can give Mega Mawile a serious advantage on the battlefield. In more desperate situations, you can attempt to force your opponent to KO one of your other Pokemon with Outrage, giving Mega Mawile a guaranteed turn to wreak havoc! Finally, scouting your opponent's team for Choice items can also work wonders, because Mega Mawile's wide array of resistances allows it to prey on many of these Pokemon.
Mega Evolving Mawile on the first turn it's out is usually a fine option, but it's not always the best one. Intimidate is a fantastic ability on its own, so if you plan on switching out right away, don't Mega Evolve. This can allow Mawile to stop sweeps in desperate situations with a double dose of Intimidate and generally put more pressure on your opponent.
Finally, one of the trickier aspects of Mega Mawile is Sucker Punch. Sucker Punch can be the root of terrifying, game-deciding mind games, due to its low PP. There isn't much that can be done about these mind games (besides running Substitute), so scouting your opponent's selection can at least ease some of this pain. For example, if an opposing Pokemon has Substitute, then using Sucker Punch can be riskier than usual.
Team Options
Thunder Wave support is an easy way to make your opponent's Pokemon vulnerable to Mega Mawile. Because paralysis spreading tends to fail against offensive Ground-types, this makes Cresselia, Rotom-W, and Porygon2 excellent choices to consider. As a special note, Cresselia and Latias can use Lunar Dance and Healing Wish, respectively, to give Mega Mawile a second shot at sweeping. If you desire a more offensive option, you can opt for Thundurus, which has priority Thunder Waves to ruin offensive Pokemon's chances against Mega Mawile. Another form of speed control that can make Mega Mawile a terror is Trick Room. While the five turns can be limiting, they allow Mawile to blast through offensive Ground-types much more easily. Porygon2 and Cresselia are the best Pokemon for this job, thanks to their bulk allowing many setup opportunities.
Mega Mawile's amazing typing leaves it vulnerable to only Fire- and Ground-type attacks, so make sure your team can cover those. Flying-type Pokemon and Levitate users are perfect for switching into Ground-type attacks, while Water-, Fire-, and Dragon-type Pokemon can aid against Fire-type attacks. These qualities make Rotom-W a fantastic partner for Mega Mawile. Not only do they cover each other's weaknesses, but Rotom-W can also deal with Talonflame and most Ground-types effectively, while Mega Mawile can switch into Grass-type attacks (though it should watch out for Breloom's Spore). On the more offensive side, Dragonite, Latios, Latias, and Hydreigon offer equally good typing synergy and can also lure in faster Outrages for Mega Mawile to take advantage of.
Mega Mawile is extremely vulnerable to Will-O-Wisp if it lacks Facade. While special attackers can be used to absorb them, Fire-types such as Heatran and Blaziken can also switch into them (though both have problems switching into Rotom-W), while Heracross is notable for using Guts to take advantage of Will-O-Wisp.
Finally, Mega Mawile's wallbreaking power can make it a useful teammate for several offensive Pokemon that are walled by Cresselia and Porygon2. Offensive Ground-types such as Garchomp, Mamoswine, and Landorus-T greatly appreciate Mega Mawile's wallbreaking powers.