Both the Titan and Warlock become invincible when they are preforming their power moves but the Hunter in completely vulnerable during theirs. I know it would be ridiculous to run around invincible with a 3 shot insta-kill gun but they should be invincible when summoning the golden gun to be even with the titian and warlock.
Neither the warlock or the hunter are invincible during their supers. the problem is just that it's nearly impossible to do enough damage to them in time to stop them. Possible fix could be to have super charged enemies names appear in orange.