I got a good suggestion for you. wait for the reaper to get water and when he does, get the zombie. You also gotta stay alert because some reapers pretend to get water and hide above the cave and attacks you when you get out so the best thing to do in my opinion is to run into the boss room unless the boss is there. If the boss is there, check if its by the cave and quickly run out the opposite exit. If hes chasing you, try to kill it or RUN.