This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
Definitely damage - a very strong hit or two will bring it down. The excess damage won't run over to harm the creature though, so if you feel the shield is about to fail you should save your strongest attacks till after.
My first hard mode run through included an Earth Mage, and the answer to your second question is "yes."
Goromorgs are easy at range, and they will strafe with your own movements to keep line of sight (and thus be able to cast on you). Knowing this, you can very often force them to move back into existing Poison Clouds (freecast clouds or the spawned clouds from Poison Bolt), which kills their shields.
The only time they were difficult is there was more than one & my team got caught in a crossfire.
