During resolving the "The vampires" task I faced with test that fails because of wrong logic in Battle.fight method. This test:

    army_1 = Army()
    army_2 = Army()
    army_1.add_units(Defender, 5)
    army_1.add_units(Vampire, 6)
    army_1.add_units(Warrior, 7)
    army_2.add_units(Warrior, 6)
    army_2.add_units(Defender, 6)
    army_2.add_units(Vampire, 6)
    battle = Battle()
    battle.fight(army_1, army_2)

Maybe you can add more tests to "Army Battles" to check that it works correctly.