Complete all levels on Insane. Including Glitterhelm bonus level.
The easiest way to do this would be in a party of 4 with characters over level 50. Try to upgrade your weapons to the best possible stat. A good weapon tends to be one with a elemental attribute attached to it so make the best of weapons with elemental attributes. Your main character should also have their "health","strength" & overall "tower defenses" at a fairly decent levels. I would suggest using a character that is a pure
Also note that with more players the overall enemy count increases drastically, you may want to take the insane attempt alone depending on your play style as it is possible. Suggested level before attempting solo "70".
See Clips of Levels on Insane for assistance.