Martian Engineer

It barely fights you directly. What it does is build, and nothing in the event cleans up what it built. Leave one working for a minute and your arena fills with stationary guns that keep firing long after the engineer is dead, while fresh soldiers keep arriving. Every piece of advice about Martian Madness collapses into one instruction: the moment an engineer shows up, it is the target.

• It needs solid floor within a few tiles of itself, with clear headroom above and no lava, so a low ceiling or a platform heavy arena leaves it far fewer legal spots. One more reason to fight this event somewhere you prepared.
• Hurting it does not interrupt construction. It shortens the wait before the next turret instead, so poking one and wandering off is worse than never touching it.
• Confusion stops it building outright, and there is a short grace period after the debuff ends before it can start again.
• The turret allowance scales with how many engineers are alive but is pooled between them, so one engineer with room to work can burn through nearly the whole quota alone.