On the challenge aggressive infiltration, on the first room, an unkillable champ golem spawned (the type with the Gatling guns). Got it down to 20% health twice on the same instance. It healed back up to 50% the first time, and on the second time back to 100% (instance done solo).
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Update, the challenge mote makes the champ golem come on Aggressive Infiltration. No idea if its intended to be unkillable though.