It's obtainable, once per character, in the quest Tyrell's Ego, by beating the quest with 100% kills (1075 enemies? I don't remember) within 1 hour (30+ minutes left on timer) on Ultimate. Not too easy and requires a decent party, but you can obtain your own. Hint: do a 4-way PB before starting the quest, and map the quest out once first. At least 2 players are required to get to all the enemies.