I used these bad boys and a Complexity -1.
There was no strategy involved, I just went in there guns blazing and made all of the Lucario combos I could. In hindsight there were some better strategies, but this worked for me first try, due to the Complexity -1 eliminating enemy Eevee. I imagine that that was what helped the most, since I just tested the Pummel strategy and I utterly failed miserably. Here are my suggestions:
1 - Go with the Pummel strategy. Lucario, Mienfoo and Hawlucha will do more damage collectively than Charizard, Torchic and Combusken (Rotom-Heat would help here).
2 - Level grind on Mawile. Defeat will still give you experience, and the higher your levels the better you will do.
3 - Save up enough coins for a Complexity -1. There are a few ways to do that. Participating in the Mega Lucario contest gives you a Jewel, as does the first completion of the Kyogre stage. If you have enough Jewels then you can buy the coins necessary for a Complexity -1. Also, if you have captured Mew on the Mew Stage, you can earn another 1000 coins for completing it again.
I wish you the best of luck in the conquest of Mawile!