Below are some strategies to defeating the Giant Jelly.
First, build an arena about 100 blocks long, with several domes about player height above the arena floor. The Giant Jelly is very large, and cannot even dream of fitting through a Player-sized door. The player can then shoot or stab the Jelly at his/her leisure as the Jelly attacks in vain. Weapons that inflict damaging status effects, such as fire and poison, are very helpful against this boss.
It is possible to dispatch the Giant Jelly easily by fighting him on a steep incline. Use the peanut butter trap at the top of the incline, and immediately work your way down. The Jelly will follow you, but his minions will not. From anywhere on the cliff, you can shoot straight up to damage the jelly. Because you are directly underneath him, he will continually try to use body slam, but will just run into the mountain. A video detailing this tactic can be found below:
The jelly has three moves that can be considered attacks.
The most common such attack involves the Jelly jumping up and then trying to land on top of the player, similar to monsters using the "Body Slam" move.
The Jelly can also shoot explosive jelly from his sides in all directions. Avoid the jelly, similar to the "Explosive Phlegm" move used by monsters, as it will explode in a few seconds and damage anyone near the jelly.
Finally, the Jelly starts the fight with two additional smaller Jellies. While not the focus of the fight, it is best to kill these smaller jellies first, as they can fit through gaps that the larger jelly can't. These smaller Jellies will try to body slam the player, and may also shoot bubbles from their eyeballs.
The Giant Jelly drops the Endomorphic Jelly , which is used to craft the Durasteel Upgrade. This upgrade allows the player to travel to the fifth and currently final sector, Sector X . It also allows the player to craft additional weapons, armors and furniture at the crafting stations.
The Peanutbutter Trap may be a reference to the popular YouTube comedy song "Jellyfish," by Julian Smith.