Make monsters that can't take damage in close range.

Imagine that. You have a char who mainly fights in melee, but some monsters are completely immune to all damage at that range. So you'll have to run away and throw shit at them from afar to kill. Isn't it genius?..
Let's call this mod "Proximal Invulnerability" and put it on all kinds of mobs. The more, the merrier.
And the best thing is that it makes zero sense. Exactly how you like it.