Awakening of Evil

In mission 4, does the Necromancer's artillery need to destroy both garrisons, or just one? In my case, it destroys the first garrison, but the second one remains, containing 3 * 900 Archers, 300 Ents, 2 * 130 Crystal Dragons, and 500 War Unicorns. Behind the garrison, Faidaen is stationed with 1.7k Dancers and about 80 Dragons, etc.