Scavenger

Scavengers are a late-game boss that can spawn in any level once the difficulty meter is around 3/4 full, making them the only boss to appear on any stage. On difficulty, they can appear as early as Very Hard. The Scavenger's attacks consist of a missile barrage, an explosive payload from their Brilliant Behemoth, and a ball of electricity from their Ukulele.

They also possess two passive abilities: their Mysterious Vial grants them a slow, constant regeneration effect, and at low health, they will activate what seems to be either a Soldier's Syringe or a Energy Cell - the effect is more in line with the latter, but the visuals are what you'd expect from the former.

Once in this phase, they attack much faster than before. Despite all the loot they carry, Scavengers do not seem to seem to have a greater chance to drop the specific items on their bodies.

Field Notes
Who knew a pack rat could be so dangerous! This creature seems to be hoarding many of the items littered across this planet. I have seen no others of its kind. If it hadn't been so intent on slaying me I would have tried to gain this Scavenger as an ally...

Will I ever see a friendly face again, or will my last breaths be on this hostile planet?