InformationFellbug is a re-textured Rock Scarab. He can go through walls and is just as slow as the regular Rock Scarab. He is the ultra boss for the Hollow Caverns and can be spawned by talking to Monster Crafter Olaf in your ship after killing 1000 enemies in the Hollow Caverns.
Fellbug has 4 possible ways of dealing damage. On contact he only deals 2 damage but that can very well kill you when you are at 1 health left since his Scarab Eggs and Projectiles he shoots both do 99. The Slimes that it spawn can fly and only do 6 damage. It is recommended to use a NulDmg Potion which reduces all incoming damage by half.
Tips & Tricks
Fellbug has 3 hit boxes (His Head, His Body, and His Tail). To maximize possible damage output it is recommended to use weapons that have piercing shots so that you can hit all of his hit boxes at the same time. His contact damage is low, but his Eggs do massive damage. The best thing to do in this fight is to try to keep moving so you don't get cornered buy all of the eggs. The Slimes are a light shade of purple and can fly. Although the Slimes only do 6 damage and have very little health, they can still be a problem if you don't take care of them soon.
It is recommended to have a full health and full stamina when going into the fight with Fellbug. You will need a lot of Health Pack III and a couple of Stamina Pack III for the fight. Use combat chips that can hit multiple hit boxes such as the Plasma Grenade. It is also recommended to keep moving so that his projectiles wont hit you and the Scarab Eggs wont corner you.