I looked at every comment and nothing helped. As a druid, I was too weak to melee them down, as I did 0 damage, and my pet was too weak as well, very rarely doing a few points. I would have had to sit at each of these for hours to get it down.
Then I learned something. These things are NOT spell resistant at all, in spite of what they say. In fact, any spell will land on them easily. If you cast a spell on them and it says it was resisted, it simply puts up the wrong message. What it means to say is, your spell failed because you cannot see your target, and we are keeping your mana. Go cry about it.
If you can see these things and are swinging at them with your melee weapons, your spells will land. You just have to find that sweet spot, described in the other comments. When you swing melee, cast a spell and destroy them. Caution: If you use dots, you must still be able to see it when it ticks to 0%, even if you had the spell land. So stay in melee range until its dead.
If you have levitate, you can strafe up to the top of these things to try and find the spot you can melee them from. I have noted several, specifically the ones on the mesa, which I could not locate that sweet spot on. If you cant find it, go to another one.
This quest definitely does NOT perform as intended, and is quite certainly still bugged, but it is at least able to be completed