Instructions
Determine which of the three vents is emitting vapors. Only one vent emits vapors at a time and after you ignite it, it will die and another vent will activate.
Stand behind the vent. Eventually, the sea monster will rear its head back, preparing to lunge at you.
Ignite the vent with your flamethrower just as the sea serpent rears its head back. If you time it correctly, the vent will explode just as the monster's head passes over it. You'll know you were successful if it shakes its head a few times afterward.
Move to the vent that's now emitting gas and once again ignite it when the sea serpent rears its head. Repeat this several times and the serpent will die.