After opening the boss door, you will enter a room with twelve skulls that may contain hearts or arrow refills. Once you go through the door up above, you will enter the battle ground. You will see a large, stone head on a stand. The head will begin to glow and will lift off the ground. The stand will disappear and two large robotic hands will appear in front of this head and move to its sides. The hands will clench into fists several times prior to the battle.
The battle starts with Mazaal lifting up one of its hands to crush you. You need to go up to the hand he isn't using and shoot the pink eyes with arrows until the hand falls. Then you can swing at the pink eye until it blows up and falls to the ground, turning that side of the head dark. Mazaal will try to grab you, so just outrun his hand. Shoot arrows at the pink eye on the other hand until it falls, then slash at the eye until that hand blows up and falls. The entire head will then shake and fall to the ground. Shrink to Minish size and go through the small hole on the head's chin. Inside, you will see six pillars, but only one will be glowing. Swing at the glowing pillar until it blows up. Once it does, a crane will pick you up and take you out of there. Once you're out, Mazaal will be back to normal, so return to normal size.
Use Arrows to cause damage
The second part of the fight will be the same, but Mazaal will attack with both hands, alternating between the two. This may make it harder to get a clear shot, but don't give up. He has no new attacks, so just shoot the pink eyes and slash them until his hand shuts down. Do it again for the other hand and go shrink yourself. When you get inside Mazaal this time, however, there will be dirt filling the room. Equip the Mole Mitts and dig your way to the correct pillar, slash away at it, and it will explode. The crane will come by again and carry you out. Mazaal will be, once again, back to normal, so resize yourself.
The final part of the battle will be the same process, but Mazaal will have a new attack; the Shrink Ray. This attack doesn't do much damage, but it will force you to resize in one of the corners before continuing. Otherwise, repeat the process of shooting his pink eyes, shrinking, and entering the head. When you are inside, you'll have to dig your way to the pillar again. Once the pillar is hit enough this time, all of the pillars will blow up and the room will begin to explode. You will be transported to the arena, right in front of Mazaal as he starts exploding.
Eventually, Mazaal will explode into a cartoony puff. A heart container will appear and the door at the top of the room will open. Grab the heart container and head through the door.