Null

Phase 1

• Grab the claws to pull out Null's arms until they become white in the middle. This will make Link focus the arm and spin attack it to destroy it instantly.
• When all arms have been broken off, attack the body with whatever echoes you have (Urchin, Darknut, etc).
• Pull the 3 arms out from the walls in the same way.
• Phase 1 consists of 5 sub-phases, alternating as directly fighting Null, pulling arms from the wall, and so on.
• In the 3rd round against Null, he will spawn void puddles, which Link won't walk over. So, you will have to move the arms you pull away from any puddles so that Link can hit them.

Phase 2

• In this phase, Null has 500HP which reduces slowly through the arena until it eventually gets set to 1 near the end. It is technically possible to kill him early by dealing a lot of damage, but generally this phase will simply be an autoscroller.
• Kill the tentacles using urchins only if you need hearts, dodge the tornadoes.
• Watch for the Octorok to get spawned in, as that is the last enemy spawn before the end of the phase.
• Null's HP is reduced to 1 at the spot in the image below, where the floor and ceiling both dip in toward the middle:

Phase 3

• Null now spawns with 5 arms: 2 claws and 3 Tri arms.
• The claws work the same as in phase 1, but the Tri arms are capable of spawning boss echoes, meaning you should prioritize destroying the Tri arms as the echoes are quite slow and dangerous.
• When the Tri arms turn purple and start spinning, Null is preparing to spawn a boss echo.
• A typical fight will be able to destroy one Tri arm on the first cycle, then the remaining 2 on the next. Once the Tri arms are destroyed, he can only attack with his claws.
• In the section with arms in the walls, use the strandtula to distract them.
• Spin through the cloud on the floor to bait it into missing you.
• If you get grabbed by the floor hand, spam urchins in a circle to damage it and free yourself.