Bosses for Spiral Knights.

1.Maybe try something like a mutated jelly.It looks really weird with other enemy attacks.(Like smashing or swinging its body)Just hurt it manually.But if you try and hurt it when its not charging an attack,spikes will come out of it and damage you.
Backstory:Gremlins tried creating jellies to help them survive.But their first experiment went wrong,and turned into a huge blob.They quickly abandoned the project,and continued making jellies.The mutated jelly still roams the land...

2.Dark Knight.(Last thread like this)Has equipment that you cant get,and they are all shadow.He can sheild and counter.Just hurt him when you get the chance.His attacks are just doing a 3-hit combo,a charged spin,and a charged blast.(He glows red for blast,and blue for spin)
Backstory:He was the 2nd knight ever to crash on the planet.The first one had been lost in the crash.Then he swore he would take his rage on everything on the planet,even other knights.He wrecked havoc in Haven.But when more knights appeared,he went into hiding.When knights defeat him,his darkness brings him back to life for the next battle.

That's all I could come up with.I hope you liked.