These are just things I've observed this morning.
Von Bon: Time Travel
If he's standing on a white clock, time travels faster, and the boss timer decreases rapidly.
If he's standing on a black clock, time travels in reverse, and the timer will increase.
When his HP gets to around five percent or so, he will start pushing everyone away from him with a tornado skill.
Also, he has a special mechanic that I thought was pretty neat.
Rewind: A shadow partner like effect will appear on the map in a place you were standing a few seconds ago from time to time, after a few seconds you will be teleported to it.
Red hat: Only blue hats can damage him, red hat wearers heal him.
Blue hat: Only red hats can damage him, blue hat wearers heal him.
Purple hat: Everyone can damage him.
The way you get a hat is by being affected by his hat-drop skill, which you need to spam left/right to survive.
He will target a member of the party, transform into a fiery tornado, and chase after them. When he hits them, he seals, so bring holy water. The trail of fire left behind him also deals a decent amount of damage.
Pierre will also teleport around and use an OHKO attack that has a delay before it activates, so if you see the animation where he's kinda' in the background surrounded by black smoke, move away, because he's going to try and backstab you with his umbrella.
Crimson Queen: Swap
She has a few different modes.
One face summons mirrors, which can seduce you. If you hit a mirror while seduced, you die and she's healed.
One face summons these black puddles on the ground, they slow your attack speed and movement. While standing on one, you get constantly sucked towards the middle.
One face has this giant red aura around it. (Damage reflect?).
Vellum does several things, which make for an interesting fight.
One involves targeting a player, tunneling underground, moving towards them, and bursting out of the ground, stunning them in the process.
If you stand still for too long, he targets you. His stun is often combo'd with him blasting a massive fireball out of his mouth, which is an OHKO.
Watch out on the ground for the golden circles, they're where he'll pop out of next.
If you see a giant gold circle on the ground, get out of it as fast as you can, because that's one of his instakill skills, where he jumps out of the ground on one side, and tunnels back under on the other.