Omni is a giant octopus that uses three giant tentacles to attack the player. Throughout the fight, Omni will slam his tentacles on the ground in the direction of the player. When defeated, Omni always drops Omni's Tentacles. As the fight draws on Omni will grow, unsure of whether it is from time taken or damage dealt, Omni grows.
During the fight, Omni will remain stationary in the center of the room and slam three of his giant tentacles on the ground towards the player. To avoid the attack, simply hug the wall and strafe either left or right when he raises his tentacles. Omni's center tentacle is longer than the other two, so strafing is necessary to dodge his attack.
If the player has Genghis's Soul, this boss is extremely easy because you can stay on the ceiling and avoid the tentacles.
Sometimes it's possible to get pushed out of the boss room through a door. In this case, Omni will not reset, and will slowly lose health while you are in the other room. When you clear the other room, the door to Omni's room will open again and you can attack him through the open door.