Prison Break

The Breaking pits you against multiple Warden Eternal bosses at once - you need two of them to die at (near) the same moment, on Heroic. Whittle both down to a sliver, then finish them within a beat of each other; a Spartan Charge or heavy weapon into their glowing back panel does it. Much easier in co-op where you each drop one on a count, though it doesn't require co-op. Do it on the same run as Harbinger if you're on Legendary.