Make a pen in deep, not too deep water, make sure the pen is in the water and not near land or going up to land. Tame a lot of piranha and I mean a lot. It'll be difficult since they come in groups in the wild and can break fish baskets with one to two hits, so becareful. Keep an eye on health and oxygen. But once in the basket they can spoil really quickly so hurry and release in the pen you built earlier. Keep collecting until you think there's a lot to do the damage. Now go into your pen and set all of the piranha on aggressive. Boom piranha pit. Now you can drop players or animals in.