So anyway, if you can mega evolve and attack on the same turn, wouldn't that make many pokemons way too strong? Again on Mawile, but Mega-Evolving then using Substitute while the opponent is switching (cause Intimidate, or choice locked or whatever) would put him in a really BAD position.
Mawile would just have to use whatever move she has that could really hurt her opponent and be done with it right there. She'll need specific counters probably. Something with a lot of bulk obviously, cause you can't rely on resistances alone.