It's great that pets get some level tiers to be more useful!
However at max difficulty, they are still EASILY killed in the last stage of the game before the titan boss simply by having two of those life sucking guys in one place.

As a summoner on max difficulty, I still have NO chance to get to the titan because they are more then twenty levels lower then the enemies I'm facing.
From what I can tell, even the core dweller has little life drain resistance with max upgrades and merely survives one life sucker by having a sheer, MASSIVE amount of hitpoints.