n large animal aquatic mindless swarm 

Perception +10; low-light vision

Skills Acrobatics +15

Str -4 Dex +5 Con +4 Int -5 Wis +0 Cha -5

AC  13; Fort +16, Ref +15, Will +10

HP 155; Immunities mental, precision, swarm mind; Weaknesses area damage 7, splash

Speed swim 20 feet

Agile Swimmer Fire jellyfish swarms use Acrobatics to Swim.

Burning Swarm [>] (poison) Each enemy in the swarm's space takes 3d8 poison damage (DC 24 basic Reflex save) and is exposed to fire jelly venom.

Fire Jelly Venom (poison) Saving Throw DC 24 Fortitude; Maximum Duration 6 rounds; Stage 1 clumsy 1 (1 round); Stage 2 clumsy 2 (1 round); Stage 3 clumsy 3 (1 round)

Bestiary 2, page 152.