Champion's Death Mask? In Azunite Burial Grounds?
Well, I can't say I have had problems with those... Actually, I'm on the same moment now with my "Dreadful Dryads" team
, that being two archers and a nature mage, all lvl 32, and I cleaned all the tombs no problem...
Are you sure you're doing everything all right? Are you using your special powers? If you do the 'hit and run' strategy, you will kill them eventually. You just have to back away somewhere in the open, where there's possibility of yet another escape. Like Darkelven said, in the rooms, he usually gets killed. But if you follow your own path out of the tomb and further, you will be able to kill them as I did.