Wouldn't you love to know how much heath your current boss has?
My suggestion is simple as hell: add boss health bar on top of the screen. Bar is green, as boss loses health, part of the bar becomes red. For Roarmulus twins we need 2 health bars (so we know which one to hit).
Would be nice to have an icon, shield, for example, next to health when they're invincible.
Its that simple! :)
http://forums.spiralknights.com/en/node/21817