He appears in a random place semi-close to the skull, so it's pretty hard to trap him in. Plus he's great at destroying stuff.

You could try building platforms up in the trees and shooting him with frost arrows. Don't try to use any other arrows, it won't work. To get the frost arrows, craft frost resistance mead, drink it, head into the mountains, and mine some of the obsidian that's everywhere while shooting drakes for their freeze glands. It only takes 3-4 flintheads from a finewood bow to down the drakes.

You could also hire a mercenary to kill the boss for you.