The trick is to run down the stairs and up the other side. Some marksmen will descend from out of thin air (there is no roof here, weird). Slaughter them. You may have to run around the hole a couple of times to make sure they all spawn, maybe even descend and ascend the steps once more. The boss will appear and teleport to the bottom of the hole. Kill him and rejoice.
That page seems to say that you can only cap the skill from two bosses while you're doing the quests they're part of - does anyone know of any other bosses that have it?