I'm almost shocked they haven't been mentioned yet, but even I tend to forget about them - they're just so ridiculously overpowered that you can usually write them off if they spawn above level 4 or so. Illusions. Total bastards. There isn't a single strategy you can hit them with that they can't ruin; anyone but a Crusader will end up far worse off from a slug fest due to their Weaken effect, PISORF is ineffectual thanks to their phys resist, Counter Fireball is totally unfair especially with Weaken, and unless you're an Assassin you'll have a hard time one-shotting them due again to their phys resist. A level 9 Animated Armor or Frozen Troll I can occasionally see the merit in attacking; even a level 5 Illusion is virtually never worth the effort.
^omfg I agree about illusions. And the orb of lusory? Basically useless unless they're blocking a lane. The boss isn't magical attacking, it converts for nearly nothing, and it grants no bonus for removing the illusions.
I play DD, Sc2, BLC, LoL, and many more. Msg me for games anytime! Dear DD Designers, you're awesome. TY! steam name: bobbywojo