Gluttony Gogol keeps swimming when Spear Break it off the cliff edge

I tried the Gluttony Gogol easy kill exploit explained here*, except the Gogol, instead of dying when it hits the water, keeps swimming back to where it would be on the platform above, it then teleports back up. I only got the exploit to work two times out of six at time of asking.

Why isn't it dying?

*Have Melia lure a Gluttony Gogol across the bridge to the cliff, then once it's near the cliffs edge, use Hypnosis on it. This will start the 'battle' proper, so you have to tell your other party members to stick to you, so they don't wake the Gogol up. Once the Gogol is asleep, approach it and use Spear Break to knock it off the edge and into the water for an easy kill.