Lose, or squat in your hovels like a coward, and it drops. For instance, the Greenskins have a meter measuring each army's Fightiness. In previous Total War games the factions played in a relatively similar way, but not any more. It's a mish-mash of everything someone at Games Workshop ever thought was cool, and it's both familiar and really weird. Lovecraft and Fritz Leiber funnelled into it through industrial pipes while copies of 2000 AD and heavy metal album covers are scattered on top. The Warhammer World is a fantasy setting, one loosely based on Renaissance Europe but with the fiction of J. Cavalry flank and race ahead to attack missile units before they get too many shots off spears defend and resist cavalry charges missile units pour volleys into dense infantry units as they slowly advance. Until now Total War has recreated historical eras, and so the tactics have been based on simplified versions of real-world tactics, whether deployed by Rome or Napoleon. Slayers would be even better as they have the Anti-Large trait as well as Unbreakable, but this is a game where paper can beat scissors so long as there's enough of it.
They win, because in the rock, paper, scissors of Total War: Warhammer the Longbeards' immunity to psychological effects makes them good at fighting fear-causing Giants.