Repeted soul burns aren't going to do any good if he can't get through the mdef. If you put this in a situation where you can't optimize your mdef, as you have to deal with multiple people at once, it's very likely that this situation would also permit SP items, in which case soul burn becomes useless.
There is that new wand that is coming out, wand of piercing. It might change things up a bit when you can pierce 10% of mdef (and maybe 1% for every upgrade, I'm not sure.) If you could pierce 15% even that gives you back like 100 damage per hit even on a high mdef person, which might be enough to turn the tables in some battles. We will just have to wait and see.
