how to be a douche in shogun 2 vanilla sieges on offense:
spam as many vanilla samurai archers as you can, have 2 bow warrior monk vets with accuracy/reload/range upgrade, and a bow hero to countersnipe their general/best units with its absurd range. have 4-6 vanilla naginata samurai as a meatshield against sallies and to eventually scale the walls (they have high armor, morale, and decent offense)
use as many bow retainers as possible.
enjoy the rage of 15+ archers firing for 10 or more minutes while you can afk and win no matter what they have