Farm advanced(80 stam) special mission wednesday and sundays(purple/yellow and red/blue manyu) or go for advanced/maybe intermediate SG whenever it spawns. If you have many low level knights you can spam 1-3 to spawn begginer SG for low level manyuus, but it's a money waste for high level knights, and daily missions give you enough low lvl ones, so dont go overboard with that.
Edit: manyus exp fall off after lvl 30 and 50(Actually, exp needed for lvl up increases, but well). using lvl5 manyus for a knight over lvl30, and using lvl20 manyus for a knight over lvl 50, will drain your money, so care about that too.