Question

What nature of a metagross is good for both physical attack and speed?

Answer 1

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.

Answer 2

It's impossible but you can just select nature suitable for your attack(such as: adamant, naughty) or speed(such asjolly or naive)(there isn't any nature good for both attack and speed)

if ur Metagross needs just physical attack, I suggest you should choose Adamant nature(it's good for ur attack but may decrease ur special attack)
hope this help