Mega Mawile is probably never going to check anything. Its defenses are poor before the Mega Transformation. With low speed, it does not seem like Mega Mawile sets up a sweep... instead, it plays the role of some sort of support pokemon on the team. IE: Mawile is a dedicated wall breaker.
For these calculations, I'm doing 252 HP / 252 Atk Adamant Mawile. Percentages are roughly in the middle of the range for simplicity's sake.
Scizor Choice-Band Bullet Punch does some ~48% damage to Mega Mawile (potentially 2HKOing, but more than likely 3HKO), and Life Orb Bullet Punch is an easy 3HKO. Mawile wins in the long term: Sucker Punch 2HKOs Scizor. Scizor may run Iron Head now for the Fairys however, which 2HKOs Mawile with Life Orb.
Conkeldurr seems like a good switch-in, but he still 2HKOs Mega-Form with Fire Punch (even after Intimidate + Mega Transformation). Mawile OHKO's with Play Rough, and outspeeds him though.
252 HP Mega-Mawile is 2HKOed by Low Sweep Breloom... and outsped as well. Mawile OHKOs easily with Play Rough of course, but this is far from the ideal matchup for a slow bulky `mon.
Dragons in general would seem like a good idea, but every dragon runs either Fire / Dragon or Ground / Dragon for coverage. Dragons have always been built to deal with Steel pokemon, Mawile is no exception. Mawiles pitiful speed guarantees to move 2nd... and since most dragons either fly or levitate, there is no hope for Sticky Web to help Mawile out here. With Fairies threatening to come in on every outrage, I doubt that players this generation will be as Outrage heavy as they used to be. Nevetheless, Mawile OHKOs Hydregeon and Standard Garchomp with Play Rough. 252 HP Dragonite with Multiscale takes ~80% damage from Play Rough. With Stealth Rocks support, not a single dragon wants to switch into the Fairy attack of choice.
However... in 1v1... Mawile takes great risks in staying in. Hydregeon, Dragonite, Salamence, and Garchomp all outspeed Mawile, and hit with Earthquakes or Fire Blast. Mawile theoretically hits these dragons very hard, but unless you know they aren't running a Fire or Ground attack (very very unlikely to me...), Mawile loses. Even in an even matchup, Mawile is OHKOed by any number of attacks.
Walls like Skarmory and Ferrothorn don't like Thunderpunch or Brickbreak respectively. Both are clean 2HKOs without Stealth Rocks. But those without access to the move tutor (or those who want the superior breeding mechanics of Gen6) are left without Thunderpunch.
Hippowdon (Physical build) is barely 3HKOed by Play Rough. Ice Punch is needed for the 2HKO. Uninvested Earthquake 2HKOs 252 HP Mega Mawile without intimidate. Mawile "wins", but the sweep is clearly over with only a single digits % hp left. Special Def. Build Hippo loses some bulk, and is 2HKOed by Play Rough.
Terrakion is OHKOed by Play Rough, but (no intimidate) Mega Mawile is 2HKOed by CB Close Combat (like 87% dmg).
Cloyster is 2HKOed by Play Rough (or OHKOed after a Shell Smash). Mawile resists all of Cloyster's attacks except for Hydro Pump, which OHKOs Mawile at +2 + Life Orb. Sucker Punch spam is the ideal way to win here, easily scoring an OHKO if Cloyster decides to Shell Smash.
Aegislash is hit for 80% damage from Sucker Punch in 252 HP Shield Form, and Mix-Aegislash's best attack is Shadow Ball for ~75% damage. +2 Shadow Sneak does less than 45% dmg for a clear 3HKO, so Mawile can stop an Aegislash march. (I'm hesitant to call a 60 speed pokemon a sweeper)
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Based on the calculations above... it appears that Mawile's best strategy is to switch into a physical wall at low risk, and 2HKO the wall, and royally screw up the next Pokemon that dares to revenge kill it with a Sucker Punch. Dragons unfortunately scare Mawile away, ironically turning the type chart upside down. Mawile's best strategy is to retreat to Scarf Salamence : who'll resist Fire Blast and Earthquake, and mess up the Dragon that dared to attack its partner. Mawile safely switches into Life Orb Starmie Ice Beam, and OHKOs with Suckerpunch. There seems to be some synergy between these two. (Thanks to the "cores thread" for bringing up Mawile / Salamence as a core. Not my idea).
Mawile is a wall breaker. Mega Mawile can 2HKO every single wall assuming perfect prediction. Play Rough (Clean 2HKO Reuniclus), Thunderpunch (Clean 2HKO Skarm), Brick Break (Clean 2HKO Ferrothorn), and Ice Punch (2HKO Physical Hippo with Stealth Rocks), and 2HKO Sucker Punch (Aegislash) all break a wall of choice. Mawile has enough bulk to mess up the switch-in attack, and still win vs the wall 1v1 afterwards. But notice: to 2HKO every wall, Mawile requires 5 attacks.
Another possibility is to run Swords Dance + Sucker Punch + 2 Wall Breaker attacks. (choose two out of Play Rough, Thunder Punch, Brick Break, and Ice Punch). If you get a good switchin vs a Choice user (ie: Choiceband Megahorn or Stone Edge), Swords Dance will lock in the OHKO against the wall that switches in... provided you have the right attack available. I haven't thought about this set too much yet however. Swords Dance + Sucker Punch offers a unique, powerful, priority attack to Mega Mawile. +2 Sucker Punches OHKOs the likes of Garchomp (without Stealth Rocks) and Magnezone, that may be what is needed for Mawile to lock in a sweep. Mawile has the most powerful Sucker-punch in the OU metagame. Yes, her Sucker-punch is more powerful than the likes of 150 Base Atk Mega-Absol, despite the lack of STAB
The swords dance insures that the "wall breaker" attacks you've chosen will turn those 2HKOs into OHKOs, but with only two "wall breaker" attacks, you will not be able to sweep all teams.
Side note: Mega-Absol seems like a better user of Sucker Punch, due to the synergy between Magic Bounce, Sucker-Punch, and Pursuit.
