Physical Immune, Elemental (your spec) Immune, Lightning enchanted?
They were a doozey on hell mode d2
The 3 bosses at the same time for the pandemonium event? ;p
I think more of an issue is most people didnt really play the hard difficulties in d2
I actually think the laser, frozen, descrator mobs are some of the more interesting ones, at least theres some level of paying attention and avoiding it that factors in (except in a tiny corridor with 4 of them in the case of lasers -_-)
Invulnerable, fast, life link on an already fast mob is a kick in the nuts tho