Trait: Intimidate (for lower attack before mega evolving) --- Huge Power (one of the best abilities)
4HP, 252 Atk, 252 Def
Swords Dance (raises mawile's attack to 2x, which counts as 4x due to huge power. this will bring mawile's attack to more than 1,2 THOUSAND, nd then you can just sweep your opponents).
Play Rough (STAB fairy type move with 90 base power, and 90 accuracy. it is like a must for mawile because a stab move with huge power... yeah).
Iron Head (another STAB move with 80 power, and 100 accuracy. just sweep. no more need to say anything).
Ice Punch/Sucker Punch (i'll go with ice punch for coverage to ground tpyes, but you can also go with sucker punch for priority. don't use ice fang. it has lower power, lower accuracy, and mawile is slow ((the main reason why you was gonna use sucker punch)), so the flinch chance is nothing).
i would be going for the moveset without sucker punch, because mawile is bulky and don't really need to attack first.