The enemies that should be killed are:

Alpha Creepers (larger variety)
Behemoths (Boss enemy)
Berserkers (large fire or electric enemies)
Creepers (smaller variety)
Cultists
Monoliths (structures that boost enemy health)
Ravagers (spike shooters)
Tentacles
Wraiths (can disappear)

Note: Berserkers may explode before you can kill them. Try saving them for last.