My Rules
Very rare1.4%ObjectiveOptionalSingle-player
Completed any Mission with 3+ Skulls active on Heroic difficulty.
Turn on three or more Skulls (the gameplay modifiers from the mission-select screen, not the collectibles), set Heroic, and finish any mission. Pick harmless ones - Thunderstorm, Cowbell, Grunt Birthday Party, Iron and the like don't make it meaningfully harder - and run the shortest mission. Nothing to it once they're toggled.