Pick two species and press fight.
Method. Durability scales with body mass to the 0.75 power and rises with hide thickness. A strike lands damage from measured bite or impact force, cut by the defender's armor and multiplied by how well that weapon concentrates it, so a jaguar's skull-piercing canine does more with less force than a wide crushing bite. Hit chance comes from the agility gap and the size gap, which is why small animals are hard to connect with and large ones are impossible to miss. The ring is locked. Nothing leaves it, so nothing routs and every bout runs until one side is finished or ten minutes are up. Nerve is not a quitting threshold, it is how hard an animal presses once it is losing: a honey badger at full nerve never lets up, while a human covers up below a third of its health, swinging less often but harder to hurt while it does. Heavy animals sweep rather than bite, a rearing swipe or a trunk or a lowered head catching everything in a forward arc, hitting harder per swing at a longer cooldown, though one blow only connects with so much however many bodies are in the way. Coming in behind the shoulder line lands cleaner than trading with the front, which is what a pack actually does. Pack animals rotate: whoever has the most left in the tank takes the engagement slot while the blown ones drop back to breathe, so a wolf pack is never all gassed at once. Only as many attackers as physically fit on a body get to swing at it, so numbers stop helping past a point set by the size ratio. Clamping and constricting weapons lose power against heavier targets, since they need leverage. Venom scales against body mass, compressed into seconds instead of hours. With nowhere to run, stamina decides most fights: effort drains the tank, an empty tank drags damage and attack rate, and the animal still standing at the end is usually the one that could still breathe. Bouts still live at ten minutes are scored on damage done.