Have at least two Medixir items, the Leviathan 3, and Vulcan 3 barrels. Use L so you can sway from left to right, keeping the monster in your screen. Take the Boss' HP all the way down until he has only a little remaining. Use the Leviathan 3 to do this, then use the Vulcan 3 for the finishing touch. You will probably have to use at least one Medixir before this point. Make sure you do not use the other. Wait until you die to use it. Do not use any Neo-Psionics to fight him; only use Cure 2 and 3 on yourself.