I've been killing these things since i was a lvl 52 druid and it's quite simple, stealth behind him in cat form, do as much damage as you can till your health is in between 1,500 and 1K, then come out of cat form and hibinate them, heal yourself and repeat if you have entangle roots it's a lot easier, and rejuvenation and regrowth is very useful aswell because it will continue to heal in cat form.

also I've noticed that killing the ironhide devilsaur takes longer because they resist or have a shorter period of time with hibinate and entangle roots, plus i think they have more armor and health then the others.