A formula would require you to decide your ATK curves first, and what damage you want them to do at what DEF values. Otherwise that is an impossible request. But once you have that much decided you can use excel and interpolation to get a damage curve that does close to what you want (I say close as no interpolation formula is 100% perfect, so some compromise is needed). Or...you could go the D and D route, where all weapons do fixed damage. For instance, a Long Sword is 1 - 12 damage on the old Gold Box D and D games. A high STR can boost that a little, but otherwise, it will always be 1 - 12 damage. Though they did let you find + weapons which added a little more damage. So a +1 sword did 2 - 13 damage, a +2 sword was 3 - 14 damage and so on. In that system, it is still possible to win with a standard Long Sword*, but a Long Sword +5 (the best they had) makes it go a little faster. *: Assuming you don't need a magical weapon to even hurt the enemy, that is.