. My A5 merc has last wish and life tap never happened the entire run.
Merc is probably struggling to hit bosses. Remember ITD from last wish doesn't work on bosses, and if you say there were few minions he might just be swinging at air, especially if he's not high level (ubers are like monster level 110? and level difference lowers chance to hit). Could try to enchant him with demonlimb. Could get an alt source for life tap.