You are doing the same mistake that I did the first tries on this boss.

Make your toons follow your pala.
Have the pala spell that stuns undead for a couple of seconds on hand. you will need it if the mana tomb is up and the skeletons respawn at the same time. Come really handy in this fight and later in the instance.
I personally just AoE nuke when mana tomb or the skeletons are up.

This boss is relatively hard at the beginning, but once you get used to him it's not that difficult encounter.