Legend of Zelda: Twilight Princess HD - Boss: Twilit Parasite Diababa

After finding all of the monkeys in the Forest Temple, the path to the boss and the Fused Shadow opens up.

The first phase of this fight is fairly straightforward. Using the gale boomerang, simply target a bomb first and target one of the heads second. The boomerang will grab the bomb and put it right into the mouth of the head you targeted. Do this to both heads to end the first phase of the fight.

Afterwards, the main body of the boss will emerge from the poison water, and the second phase of the fight begins. Ook the Baboon will appear a few seconds into the fight, and begin sliding left and right across his zipline while holding a bomb. The idea here is to use the gale boomerang to grab the bombs from his hands and hit the boss with it.

This part can be extremely annoying if you aren't precise with your timing, and I usually get stuck here for a while because the boomerang will simply bounce off of the boss if you throw it at the wrong time. I figured out that you have to AVOID z-targeting the main head (or else it will lock on) and target the bomb FIRST instead. When Ook reaches one of the side walls, manually target the bomb first, then the head, and throw the boomerang right before he jumps; this will cause the bomb to hit the side of the boss's head, stunning him and opening him up for a sword attack. Rinse and repeat.

After he's dead, you obtain the first Fused Shadow out of three.