Bomb traps can severely reduce health levels, or kill minions outright. How about a new HEAL spell that could eventually be upgraded to RESURRECT...?
The stipulation is that a warlock needs to be in play to use the spell. Heal levels could be low percentages at the first level and get larger with upgrades. You could use the spell more than once to increase the healing of still living minions. It should even work on the immortals.
One of the higher upgrades could be that after minions have been healed, they are impervious to attack from traps and room defenses for 5 seconds.
Once you reach the resurrection level, it can resurrect all dead minions that weren't already turned into skeletons by the necromancer, but won't revive skeletons either. Also, it can't ressurect immortals, but one use of the ressurect spell could put them back to full health if they're nearly drained of health but still conscious, not stunned, but only once per dungeon attack.