I suggest mega kangaskhan because it has the parental bond ability which can do something really powerful. Here it is.
Kangaskhan with the nature brave
Moves: fake out; for flinching plus 1.5x damage
Power-Up-Punch: double attack boost
Aerial Ace: to take care of fighting types
Outrage: use it for sweeping or when your just about to give up
If you don't have kangaskhan, you can find it in glittering cave(it's slightly uncommon, so it might take you a while to find one) Mega stone is also in glittering cave, but apparently I forgot where it is in the cave(you need the upgraded mega ring to get it though)
Hope I helped!