This is what I was concerned about: enemies that are immune to both of my magic skills, and regenerate so fast my merc can't do any damage to them. It means I have to lure the monster to a corner, and teleport away quickly.Not fun, and not something re-arranging my stats/skills is going to solve, I think. Time to do some research. Looks like there's not much I can do about it. There is a rune combination that I can put in a weapon for my merc to use, that will reduce enemy immunities, but it only works in Battlenet, and I'm playing the home game. There's nothing more to do but max out cold and lightning skills, and find the best equipment I can. So I'll go at least a bit further before deciding whether I'm done here.
