More a case of balancing characters for game play.

Lore wise, omni man would be on par with elder gods and maybe some of the titans (like kronika and geras, maybe shao khan at his peak would be a good fight). Geras would be a good match as he can literally control time, he could in theory just age anyone to dust.

Obviously, normal humans like Sonya blade and kenshi would just be splatted like a bug against a windscreen.