As said above, Adamant is a great Nature for Metagross and since he has low Speed, Brave is another great Nature. Here is a set...
Earthquake- for Fire types
Zen Headbutt- more STAB
Ice Punch-for Ground types
For Lucario, I guess go with a Bashful Nature because it doesn't Raise or lower anything. Here is a set for Lucario
Aura Sphere-STAB and for other Fighting types
Water Pulse- for Fire and Ground types
Keep Blaze Kick for Coverage
Iron Defence/Ice/Thunder Punch-Raise Defence or coverage
Hope this helps.......