Demon

A  is one of the last bosses you will encounter. It is the fastest monster in the game with the same speed as a Demoness. It will chase you on sight and will shoot fireballs at you.

Location
You can find the Demon at the bottom of the map. It is located in the Great Cave.

Strategy
Use the Jetpack to keep at a safe distance. Use the Particle Sniper Rifle and or the Particle Shotgun to kill it. A Health Regeneration Potion is advised. Also any other potions will help.

You can defeat this boss fairly easily using the Particle Sniper, ZF-0, and Jetpack, provided you can get a bit of luck. If you use the sniper and the jetpack to lure him to the far right of the chamber, you'll eventually run into the volcano. Tag him with the ZF-0, then sit on a basalt shelf where he can't hit you with fireballs. It will take some time, and you will still have to deal with the lesser enemies, but you don't have to worry about things attacking from behind you, and if your shelf is high enough, you don't have to worry about hell hounds.