have fixed damage,
add bonus damage depending on how much you beat AC.
I.E:
Greatsword, base damage 8, +5 STR
damage 13.
Hit: deal 100% damage, 13 damage
beat AC by 5, deal +50% damage, 19 damage
beat AC by 10, deal +100% damage, 26 damage
beat AC by 15, deal +150% damage, 32 damage
beat AC...