Speak to a mysterious figure (Ericsson) on the South Freeport dock ( -297, -58, 201 ) /waypoint -297, -58, 201
After talking to the mysterious figure you will be transported to a solo boat instance.
Speak to Kain
"Kill" Lucanic Knight Helmark (even level, one up). Since he does not actually die, keep yourself alive. When he goes down the first time, heal yourself and prepare to keep fighting. You can't unmentor. I tried and the game won't allow it.
Lucanic Knight Helmark will get back up - when you get him near death, he will throw your targetting off. Chloe will throw fire near the stern's steps.
Run Lucanic Knight Helmark into Chloe's fire - He will burn and rise again, this time slowly stalking you. Kain will summon a leviathan.
After Kain summons a leviathan, walk Lucanic Knight Helmark towards the bow of the ship (that's the front), staying beyond his reach until it is fully summoned. Run to the bow of the ship and wait until the leviathan eats Lucanic Knight Helmark. He had to be at the bottom of the stairs to be eaten by the leviathan.
Fight each of the leviathan's four tentacles (even level and con or Lucanic Knight Helmark's starting level) down to nil. Tab targeting does not work, use the mouse and start at the front of the ship. The Spitters have ranged, but hit weakly. Ranged attacks and staying near the front of the ship for the remainder of the fight should make this much easier. I had to finally "kill" the tentacles at the stern to get the leviathan to leave.
(NOTE: I had to make targeting macros to survive this on my gimpy 83 Warlock. Targets are Gazer, Gorger, Spitter all have last name Tentacle.)