I'm most familiar with the DnD system of attacking, but that may be too complicated...with all the levels, base attack bonuses and what have you.
I'd say that you could use a somewhat diluted version of that system so that fights are fair. For example, a person simply picks their ability scores for Strength, Constitution and Dexterity. We'd have to flesh out how that would work precisely. But that would help you determine a character's strength with their attacks and their ability to avoid attacks.
You can use dice rolls to determine if attacks hit or how badly a person's damaged.