Uumuu

Uumuu is a jellyfish-like boss similar to the other enemies in Fog Canyon, but much larger. It is found in the Teacher's Archives, protecting the dreaming Monomon the Teacher.

In battle
Uumuu randomly floats around the room, occasionally using one of two attacks: Uumuu cannot be harmed at first, as it is protected by the bubble-like transparent shield that surrounds it. However, Quirrel will appear to help the player, leaping around the battlefield and occasionally popping Uumuu's shield. This will deflate Uumuu, leaving it open to attacks for a few seconds.
 * Creating bursts of lightning in random locations. The Knight must stand in a safe spot before the bursts fully erupt to avoid getting hurt.
 * Creating bursts of lightning at the Hollow Knight's location in quick succession. It is imperative to constantly keep moving as long as this attack is going on to avoid getting caught in the lightning.

Tactics
Although Uumuu may only have two attacks, they're both quite hard to dodge, and their pattern leaves few opportunities for healing. In addition, Uumuu's erratic movements and the very sparse opportunities for hitting it compared to its health pool result in a very drawn-out battle.

Get in as many hits as you can when Uumuu's shield is popped, and be prepared to dodge an attack right after. In desperate times, this can be an opportunity to heal instead. Avoid jumping in the water below a platform in the arena, as this greatly slows your movement. As is the case with most boss fights, Quick Focus is highly recommended. Soul-generating charms like Soul Catcher/Soul Eater and Grubsong are also very important in this fight, as chances to hit Uumuu and consequently to gather the Soul you need for healing are quite rare.

Trivia

 * The Uumuu fight is one of two fights in the game where an NPC will come to help out. The other is the Traitor Lord fight, where Cloth can help.