Normally, you should be able to stand on one side of the 'pit' and plug him with as many arrows/spells as it requires, as he's not smart enough to realise he can jump down into the pit and get to you. However, sometimes he will drop down into the pit to kill a rat or crab down there, in which case you can lead him back to the previous level and either squash him with the log trap, if you haven't triggered it, or jump down to the bottom level and shoot him from there - because of the log bridges and the fact that the drop is deep enough to injure him, he will just stand there like a lemon while you kill him.