The key to face the Evil Empire of Boi Galla is to have a decent number of pikemen (units who can form phalanxes). As the swêbóz, I used their phalanx-unit to "bog" the enemy down along a thin but strong line, and sending bastarne falxmen around the flanks and charged their backs. This way, I managed to defeat both the garrison and relief in the same battle as the reinforcement arrived too late to pose any greater threat. It was still helluva fight though, facing the armies one at the time...