The unit with the best price-to-performance ratio.

Even with 20 against 10, I'm not entirely sure they'll win in battles against archangels, gray bears, titans, black dragons, and hydras, which can only be defeated with wait.

20 against 10 will be a complete slaughter! It might be difficult against gray bears, but I think they'll crush everyone. One attack from the phoenixes will deal significant damage. After the second attack, there won't be anyone left to fight. And considering that 4 phoenixes will resurrect, it's game over.