Battle Royals (pvp) usual come down to crowd control, dpr, hp, resistances, and saves. Even mass hold person (cast above 2nd level) can win battle royals if your opponents don't have good wisdom saving throws or spell resistance.

If it's everyone vs everyone at the same time, one strategy is to find a way to hide while everyone else knocks each other out. If traveling to the ethereal plane isn't out of bounds, spend the first half of the match there. Or even a big fog cloud and hide can do the trick.

But neither of those strategies are very fun... Pvp in general isn't nearly as fun as it sounds. You'll spend more time arguing about the rules and reading through the books than you'll send fighting.