Gleeok (Phantom Hourglass)
Once Link enters the boss chamber, he will hear the sound of rushing water. Then the two-headed dragon will emerge from underneath the water beginning the battle. Gleeok's heads will take turns when attacking. The left head will spit out some fireballs at you while the right one goes with snowballs. The attacks are quite damaging but are easily avoidable since they don't track Link. So simply just move out of the way when one of the heads attacks.
You will notice the four pegs that surround Link on the platform he is on. Once one of the heads wiggles, signaling it is ready to attack, use the Grappling Hook to stretch a rope between the pegs. This will allow Link to protect himself as well as bounce the attack back at Gleeok. The trick is you need to bounce the fire attacks at the ice head and the snow attacks at the fire head. Angle the rope from the bottom-left to top-right pegs to bounce it towards the fire head. Then angle the rope from the bottom-right to top-left pegs to bounce it towards the ice head. Note that bouncing the fire attack against the fire head and the snow attack against the ice head will inflict no damage at all to Gleeok.
The Gleeok heads are quite wise and will adjust their strategy as they see Link using the Grappling hook. Once Link stretches the rope, one of the heads will stretch to the rope and bite it apart. You can't prevent the head from doing this, but you can dodge the attack by staying to one of the bottom corners. You must time your rope placements. Once one of them wiggles indicating they are about to attack, quickly stretch the rope so the other head doesn't have time to adjust. Gleeok gets a bit impatient and smarter as the battle carries on. Another one of their attack methods is that they will dive underwater and create a title wave, followed by ice chunks that are thrown at Link. When this happens quickly tap one of the bottom pegs to jump onto it. When the ice gets near, hop off the peg avoiding the ice. You can also just strike at the ice with your sword.
Three hits of the opposite type against the two heads will break both of Gleeok's masks. Now that they are angrier than ever, the Gleeok heads will bite off the two front pegs. The two dragon heads will take turns inhaling deeply and then exhaling a cone of fire or frost at Link. Run from side to side to avoid these attacks. Just before one of the heads gears up to exhale fire or frost, it will stick its tongue out as it is inhaling. This is Links chance to use the grappling hook on their tongues. Just connect the grappling hook to one of the pegs and the other end to one of Gleeok's tongues. Doing so will bring the head crashing down to the platform. Now use Link's sword and slash repeatedly at the head. Repeat this process until both of its heads can take no more.