Uses this effect:
Geiger's respawn time is two hours long.
Geiger is a level 27 enemy who is located in an area that irradiates you permanently. Pull him away from the radiation cans around him or bring some rad resistance to help avoid most of the damage from the encounter (From: Lucifer Planet FE Achievements). The radiation will also damage your vehicle or mount, so finish the last few meters of your approach on foot. Ranged attackers should hit him from as far as possible with a medium or heavy rifle as he stands at the center of his radiation-protected circle. He will attack you by slashing with his melee skill. Run around his circle, and shoot him with light or fast-medium rifles, adding some actions like Disrupt or Precise Hit 1, and so on. Don't draw him too far away from his circle (max. 25-30 meters), or he will run back to his honeypot to regenerate his health. If you die during the attack, you will respawn nearby at the LifeNetPod in Redfield. Go back, and continue your attack; he will not restore his health upon your death.
Personal: At Level 42 as a ranged, armored attacker with some food available, I needed 3 attempts to kill him. At the beginning of the third run, he was down to 20 health points. You could kill him alone at a higher level, or take a team with a tank and 2 or 3 damage dealers!
Location on the Map
Geiger from Far Away
Killed the Boss