Instructions
Wait for one of the dragon heads to come forward slightly, which indicates which head will fire. Use your grapple rope to connect two poles diagonally, so that you bounce the fire or ice ball into the rear head, since fire head is weak against ice ball and vice versa.
Jump on top of the grapple poles when both heads go underwater to cause a tidal wave. Do this five or six times, paying attention to the timing, since the tidal wave attack can take away much more life than Gleeok's regular attacks.
Attach one of Gleeok's heads to either of the back poles when it lunges onto your platform to stun the head momentarily, allowing you to attack the head with your sword. Repeat this a couple of times for each head to defeat Gleeok.