Cirno
Ability #1: Tidal Wave
Description: Fires 9 columns of shots around the boss for 10 seconds. The boss cannot move or use other abilities while this is active.
Ability #2: Absolute Zero
Description: Waves of ice spawn from the wall furthest from the hero. (long axis) Can be tricky for the hero to avoid.
Ability #3: Hailstorm
Description: Hail randomly falls from the sky for 15 seconds. This ability alone might be why third person is recommended for heroes. A warning sound is played to the hero and the boss can't move during this warning period.
Ability #4: Baka Shot
Description: Fires projectiles in the direction the boss is facing. These projectiles move around chaotically, and spawn in different sizes and colors. The boss can move around while using and even use other abilities while this one is active.
Ability #5: Frozen in Time
Description: Immediately freezes all friendly projectiles on screen. They will resume moving in 3 seconds. They are harmful while frozen.
Ability #6: Flash Freeze Warning
Description: After 8 seconds, freezes all friendly projectiles on screen. They will resume moving in 3 seconds. They are harmful while frozen. Thanks to the delay, this version can be combined with Tidal Wave and Absolute Zero.
Flandre Scarlet
Flandre Scarlet has three phases. These phases dictate which ultimate she can use. (her ultimates are abilities 5-7)
Ability #1: Persistent Pursual
Description: Fires a bunch of projectiles with "lazy homing", they briefly home in on the enemy, slowly grind to a halt, and then "home in" once again. Will easily hit a stationary target. Projectiles are protected from going out of bounds and will despawn after a set amount of time.
Ability #2: Flawed Diamond
Description: Creates a formation in the center of the arena of 5/8ths of a diamond. Combine it with other abilities to make it difficult for your opponent to navigate across the map.
Ability #3: Silent Stalker
Description: After a delay, projectiles start spawning closer and closer to the hero. The hero needs to make wide movements to not allow them to catch up. These projectiles (after their own delay) fly straight at the hero.
Ability #4: Lavatein
Description: Creates a beam wall in the direction she's facing. After a delay, the beam wall becomes harmful and rotates around the boss. It alternates between counter-clockwise and clockwise. The wall also fires rapidly moving projectiles. (it is meant to be feared, and she gets it in all phases) Boss is immobilized while using.
Ability #5: Violent Prison
Description: Phase 1 ultimate. Spawns a grid of beams which fire projectiles from random locations. Easiest to avoid from above. Here's a
screenshot. Boss is immobilized while using.
Ability #6: Four of a Kind
Description: Phase 2 ultimate. Creates three AI clones of herself and fires targeted radial shots at the hero. Boss can move while using, which is probably why this is arguably her most powerful ability.
Here's a screenshot.
Ability #7: Q.E.D. Ripples of 495 Years
Description: Phase 3 ultimate. Fires targeted radial shots from her and from random parts of the map. Can be tricky to avoid due to the randomness, which makes it hard to rate this one against Four of a Kind. Boss is immobilized while using.
More boss descriptions will be added as more bosses are added. I'll list community bosses and links to them if they're not just reskins.