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

20 Phoenixes can indeed defeat 10 Behemoths, but only if they attack first. And there will be 8 left, not 11. Besides, everything depends on the heroes' stats, magic, and many other factors. If the Phoenixes attack the enemy immediately in battle, they will be separated from the rest of their troops, while the Behemoths will be supported by Ogres, Wolves, and Rocs.