The most generic strategy is “do what you can do better than them and their natural predators.” In virtually any situation, that’s going to be tool use. You may be entering without a weapon, but sticks, stones, sand, doors, trash cans, the clothes on your back or anything you can get your hands on can put you at an advantage. You’d have to be in a pretty sterile arena to truly have nothing available.

If you’re particularly fit, you may be able to best most animals in a test of endurance: do everything to delay the clash and keep them moving, and they’ll grow hot and exhausted faster than you will.