When jumping on the red trampoline to get the boss key, hold right and you will end up on the solid ground on the middle floor.
Stand between 2 torches and make sure they are both lit at all times and attack the poes with your sword.
For the first phase make sure to attack both of Gleeok's heads somewhat equally. For the next phase, jump when Gleeok is going to land and attack him with your sword.