I was thinking of a special challenge where only black skull ranks can do it. The challenge would be just like a normal insane campaign mission except a few things. All enemies are even tougher. You can't revive, but instead use your other mechs in it's place. And there is a final boss: Godmode.

This godmode isnt normal though, he has better stats.
Such as it's HP of 2000.

To get into the challenge, you need a skull key. Skull keys come from silver and golden item boxes. As well as an extremely rare drop from regular item boxes.
Upon beating this buffed Godmode, you can get one piece out of many godmode equipment.