I ran mega-Mawile for a a good few hundred battles in-game. It's good, no doubt. It even managed to sweep four or five opponents on occasion.
Fire Fang was by far the best choice for the third attacking slot, and arguably even turned out to get more use than Rough Play or Sucker Punch, owing to the types of mon that others switched in. Substitute was better than Swords Dance.
But, in the end, it just wasn't great. M-Mawile relies too much on correct prediction, frankly, and is too predictable itself. Get the Sucker Punch wrong and you've wasted a move. Worse, you've wasted a move that somebody has switched a check in. And it's so slow. There are too many opponents it has a problem with. And even with Huge Power, it gets surprisingly few OHKOs. Fairy and Dark aren't supereffective enough. Fire Fang is only 65 base power. Finally, there are too few surprise sets. The opponent pretty much knows what you're running.
So I don't run M-Mawile any more.