Gigazapper

It only appears during Martian Madness, and that event only becomes possible once Golem is down, so there is no way to run into one early. Within the invasion it is infantry. It walks along the ground with the rest of the wave while the flying units work on you from above.

Its spear barely reaches past its own body, so the whole fight is whether it gets to you. If it does, the shock lands and Electrified starts draining you, and that drain accelerates while you are moving. That is a nasty pairing in the middle of an invasion, where standing still is not really on offer with saucers overhead.

Kill it at range or push it away. Ranged weapons and anything with real knockback keep it outside its own reach, which is all it takes. If you do get shocked, get behind cover and hold still until it wears off rather than sprinting for safety, because the sprinting is what does the damage.