That's a tough one. It seems you really have to use something to lure him off or kill him. I might be able to set a post for him.
Luring won't work. The problem isn't that he won't TRY to climb down the ramps, but he's just too big and awkward and can't maneuver himself to the point where he can make it down.
If the ramps could be made wider, or if one could be placed so that he only has to walk in a straight line to get back down, that might work. If you could confine him to the water, but make him unreachable from the land (so that anyone killing him has to go down and fight), his regen should stop people running in and out to wear him down.
He also gets stuck at the trans to the path with lots of ramps and orcs (can't remember the name atm, Redhorn mts. I think), and can't make it back down to the water (though not for lack of trying).