Stupid pigs can levitate. How do I know? Well I got ambushed by some that jumped off a cliff onto me, so I grabbed my pteranodon once I respawned and flew over. I couldn’t find them so I dismounted to get my stuff just as the same pigs jumped off the same cliff that they had previously flew off and couldn’t get back up. I got my tapejara and flew back to find them again on top of the unclimbable cliff. I got them that time.
A Yutyrannus is by far one of the best ways to deal with these guys. You can easily scare these guys away from you. It also seems that when they are scared they can't activate their heal ability which makes it easier to pick them off. Another good method is to use a shotgun. It deals huge enough bursts of damage that they can be close to death in a few shots, good for if they show up unexpectedly while you're on your own.
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.