LASO Master for the Master Chief Collection. Halo 1 and 2 both have some absolutely insane levels that took my friend and 4+ hours to beat. Gravemind in particular takes the cake since we got soft-locked at the prison elevator with no shields and had to abandon a 3 hour run.
Aside from MCC, I also did SL1 playthroughs of all 3 Dark Souls games where I beat every boss. Granted my rule was that the bosses just had to die, so if there was a way to cheese a fight, you better believe I took it (coughManuscough). Though this mostly just happened in DS1. Still beat bosses like Kalameet, Fume Knight, Ivory King, Nameless King, Midir, and Gael legit.