There's a glitch with the battle where the unhelpful spawn becomes untargetable for drain health or any other sacrafice minion spell.
This happened after using drain health on it once (I think the boss had a myth minion out but that's it) and then the second time it spawned it was untargetable to any minion killing spells this time, which made doing the battle with 3 people way more annoying than it should be.
