During a boss fight:(Ref. Difficulty Level is 0.2)
If your rexes are above 10k health and have 280% damage,they are able to crush every boss before they sacrifice.
I repeated the boss fight many times.Yuty and Daedon had 50% of chance to die,and not one rex died.No rifle or rockets,pure tames fight.
They appear in packs of three around the entire snow biomie.
Find an isolated pack away from wolfs, Carnos or Yutyrannus and pick the, of with a powerful gun, note that they don’t heal when at a decent range from the player, once they are about two strides from you they will start to heal themselves.