The Chosen Ones

Here's an idea you could have a boss summon a shadow that follows Right behind you and tries to touch you, if it hits of course you die. This forces the player to move away from it until it expires.

Another idea is a boss spawns some little adds that head towards it if they make it to him it heals him a bit, during this move the player can still attack the boss (which of course isn't smart). Maybe spawn like 6 or 8 of them on different sides each healing about 10% Boss health.

Another classic idea is to have a boss with 4 parts of a boss that cant die until each part is taken down. Then have 1 fragment of the boss occasionally toss out heals which you have to damage it a certain amount to interrupt. During another phase a different of the 4 parts could cast a nova spell with the same mechanic that would kill you.

Another boss mechanic that is best when you have all 3 of the toons involved is to make illusions of all your heroes working for the boss with some % of your HP but full dmg, consider giving each of the mirrors 1 spell from your spell list, not a stun.

Oooh this is a really fun idea, make maybe the vampire boss do a bite effect that buffs attack speed but does a big DoT as well. And for the heck of it have it give a 100% crit chance to do 1x damage so you can see dmg going over your heroes head.
Edit: You survive by doing a lot of heals on the hero who has it until it wears off after maybe like 10 seconds.

Another boss idea that is a little harder to implement is you have a boss that CANT be killed normally, but after you do X damage to it, it becomes stunned and you run through a small gauntlet behind the boss that becomes open and damage it's heart or power source maybe like 25% then it teleports you back to boss and you resume.

And my last idea for now is have 2 bosses that are like twins and you have to keep them away from each other or they do bad stuff, this is more a general boss idea not their super move.