Solo'ed the level 54 Devilsaur as a level 51 BM specced hunter.

Just keep mend pet on your pet at all times, when your pet's health gets low run up and use scare beast on devilsaur. heal your pet and wait out the full 20 seconds. after another 10 seconds of fighting, if your pet gets low on hp again, just use scare beast.

very easy. :) (as a hunter)