When entering the room, the lights go out and Greebler appears in the middle of the room. Greebler is a giant anglerfish and also the only source of light in the room. Greebler will occasionally turn off his light leaving you in complete darkness. Greebler will then attack you by either charging at you or launching blowfish at you.
Try to move around the room while avoiding Greebler. When he launches the blowfish move away from the walls. Also try firing your weapon to generate light if you cannot see Greebler. Additonaly, you can poison Greebler if possible, making him glow as long as he is poisoned.