To unlock the Burnt Out Anarchy achievement simply use Rampage mode to beat every boss in the game. Make sure that you get the Rampage +10000 bonus. If you don't get that, it means you didn't beat the boss properly.

Embark on an inspiring mission of human connection beyond the UCA. Sam — with companions by his side — sets out on a new journey to save humanity from extinction. Join them as they traverse a world beset by otherworldly enemies, obstacles and a haunting question: should we have connected?