Little William Marshal

Win every contest at one Grand Tournament with three-plus contests. Rig it shamelessly: host the tournament yourself, pick exactly three contests, and choose only one-vs-one types (board game, duel, wrestling, joust) so your own high-stat subjects fill the brackets and concede to their liege. Bring elite personal prowess anyway - some opponents won't roll over. Historical note: this was genuinely bugged before patch 1.9.2 (only the campaign's first-ever tournament could count); it's been fixed since, so no workarounds needed nowadays.