Basic Enemies - Alphabetical List:
Alien Infantry (debuts in level 6)
Suggested Attack: automatic shotgun, pulse rifle, Tek Arrows
These four-armed fiends tote deadly electromagnetic weapons, self-destruct defenses, and even jet packs. Fast-moving and unpredictable, Alien Infantry are some of the toughest foes you will encounter. Their fighting spirit is so great that even as they perish, they keep shooting.
Ancient Warrior (debuts in level 1)
Suggested Attack: auto shotgun, minigun, knife

Hulking yet dexterous, Ancient Warriors are considerably tougher than the Campaigner's Soldiers. Requiring two dead-on knife slashes or considerable attention from a pistol or assault rifle, Ancient Warriors are a grave foe indeed. Some of these warriors are armed with blowguns, which are surprisingly powerful. Be prepared to deal with these foes in droves.
Attack Robot (debuts in level 7)
Suggested Attack: grenade launcher, quad rocket launcher
You'll be pleased, no doubt, to hear that things don't get much worse than Attack Robots. These hunkering, heavily armored automatons sport miniguns and impeccable aim. The thicker golden attack robots also use rockets against their victims. Be prepared to pump as many as six grenades into one of these mechs. Explosives destroy attack robots completely, while conventional weapons will render them inoperable at best. You can then use their massive frames as cover from enemy fire.
Beetle, Giant (debuts in level 1)
Suggested Attack: knife
Common sense will beg you to stay as far away from Giant Beetles as possible. These disgusting denizens are certainly not worth your ammunition. A single pass of your hunting knife will put these filthy things to rest forever, so Giant Beetles should not pose much of a problem.
Campaigner's Sergeant (debuts in level 1)
Suggested Attack: auto shotgun, grenade launcher
Thickly armored yet a little too foolhardy for their own good, Campaigner's Sergeants are armed with pulse rifles, which they're more than capable of firing. Learn to spot these large humans before they can unleash their tell-tale energy attacks, and eliminate them as fast as possible. Though powerful, Sergeants are slow on the draw and will not work very hard to avoid your attack.