Eh, Mightyena isn't great. It's a very low tier mon that's outclassed by basically all higher physical sweepers and dark types like Bisharp. If you want to use it, I'd recommend teaching it Sucker Punch and Crunch to replace Fire & Ice Fangs and use it in lower tiers like PU, as it won't do that much work against a decent player in OU/UU/RU. As for an item, Life Orb is a definite, and the EVs should be 252 Atk / 4 Def / 252 Spe.