I found another way while i was bored doing quest I've haven't completed. Upon summoning Terokk(Terokk's Downfall - Quest - World of Warcraft) and damaging him he started to go immune at around 20%. Which got me thinking this is a great way to level up your skills as long as you have a healer if your a non healing class. This mobs doesn't hurt for much and will not disappear.
Note: as long as you don't kite him on top of the flames he will remain immune.