The thing about Mega Mawile counters is that they tend to be easily worn down, and more importantly, they tend to be really bed Pokemon in general. Just going down the list...
Torkoal: Cool Pokemon, but a really bad Pokemon :( Easily worn down by SR and Maw's Knock Off.
Mega Manectric: Even with Intimidate, +1 Sucker Punch knocks it out:
+1 252+ Atk Huge Power Mega Mawile Sucker Punch vs. 0 HP / 0 Def Mega Manectric: 297-350 (105.6 - 124.5%) -- guaranteed OHKO
It's not even a 50/50 because Mega Manectric runs four attacks. And -1 Play Rough destroys it:
-1 252+ Atk Huge Power Mega Mawile Play Rough vs. 0 HP / 0 Def Mega Manectric: 223-264 (79.3 - 93.9%) -- 43.8% chance to OHKO after Stealth Rock
Even without SR down, Sucker Punch closes the deal. Sadly, Manectric isn't even a check.
Donphan: It's a mediocre OU Pokemon, but even with max/max+ defensive investment, it gets run over by Play Rough:
+2 252+ Atk Huge Power Mega Mawile Play Rough vs. 252 HP / 252+ Def Donphan: 352-415 (91.6 - 108%) -- 87.5% chance to OHKO after Stealth Rock
Again, it's not even a check, sadly.
Rotom-H: Usable in OU, Rotom-H is in fact a Mega Mawile counter. However, like its brother Rotom-W, it's easy to wear down, which is made worse by its weakness to Stealth Rock, and unboosted Knock Off cripples Rotom:
252+ Atk Huge Power Mega Mawile Knock Off (97.5 BP) vs. 252 HP / 252+ Def Rotom-H: 137-162 (45 - 53.2%)
Between that and Stealth Rock damage, it's something of a one-time check. That being said, it definitely qualifies as about as good an answer to Mega Mawile as there is.
In general, it kinda blows that almost all Mawile checks are weak to Stealth Rock, so if you can adequately pressure you opponent, you can run those checks off the field with good play.
:(