My first group was a Priest and 4 Mages. They only had trouble with the Shade of Erenikis in Sunken Temple, but that boss was a good 3 levels higher than the others. After that I did not run into problems until that second hellfire instance, The one where you pull the lever and it spawns a bunch of waves followed by a Beholder boss.

Then I switched it up to a standard paladin 4x shaman group. Never had problems with an instance again. Dont get me wrong, I still died and had some wipes, but it was from going to fast or it was still learning the fights.