Actually, Technically, a pokemon can have 2 Natures. Though One Raises the Stats By 10%, Proud Of Its Power, Etc Alos Raise Stats a little bit, maybe 4 or 5%. So I Would Get A Jolly Metagross, thats Proud of Its power, Hates To Lose, or Etc. Teach it:
Meteor Mash, Earthquake, Zen Headbutt, and Something Else, Whatever ya want, maybe Agility to double his Speed.