How do you defeat the gnarly montster?

The entrance to the Hydra cave is underwater in the maze in Poseidon's realm. Swim across the top, down the right side, and then left across the bottom. The "gnarly monster" is the five-headed Hydra....

Nobody, all you do now is do missions and get new friends

Electric/Fighting type moves

depends on what you are trying to defeat

psycic Pokemon are weak against ghost type moves bug type moves and dark type moves, you can use ghost, dark and bug moves to deal quite alot of damage