when you enter a co-op session, your character gets the host's mission with the current progress made. if you then leave the session, your character keeps the mission. i have a character already set-up at the boss, so i make it the host and join with my new character, then return to the menu. this puts the new character at the boss. you then just complete the mission, and since the first 6 missions have a fixed boss, you get that one instead of whatever the original boss was. this is a separate category from the regular Any% because it used an already created character for the exploit, which many would (very understandably) consider it to be unfair or invalid, but since it does beat the game (and is in itself a boss rush, which could be interesting to run), it's its own category.