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. 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. Stamina runs down with effort and drags both damage and attack rate, which is what ends most real fights and why a crocodile fades on land. Clamping and constricting weapons lose power against heavier targets, since they need leverage. Venom scales against body mass, compressed into seconds instead of hours. Animals below their nerve threshold break and run, except the ones that famously do not. Bouts still live at ten minutes are scored on damage done.