I don't know if this has been posted before, but i'd just like to share with the ones who don't know.
On the first set of islands after met zone, a monster called water devil spawns every 13 hours after maintenance, and i think it respawns every 11 hours after it is killed.
This monster looks exactly like the rest, but has a bigger hp bar and does huge physical damage. I would suggest a high level trojan with nice m-def to try to kill this monster.
A picture of the islands that it spawns on is attached.