PReDS
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Then what will you use to fight the dragons from the Magma Clan? The Phoenix deals approximately 25% less damage. Four Phoenixes with an attack of, say, 10 will take down a Red Dragon in one turn, but they probably won't be able to defeat a Black Dragon. To replace the effectiveness of four Phoenixes, you would need about 20 White Unicorns or around 70 Dwarves.
The idea of capturing the Magma Clan was also mentioned on the second week as a possible further development, if capturing the Dragon City is complicated by certain circumstances. Phoenixes can easily deal with the roaming mages, but I wouldn't use them to storm the Dragon City: they won't be very useful there and can bring much more benefit by roaming the map.
4 Phoenixes per round. Phoenix: attack 12+10 from the hero, damage 20-40, per round: defense 13, 250 hit points. If you apply Bless, the damage = 4*40*(1+(12+10-13)*0.1) = 160*1.9 = 304, so it will indeed kill. Without Bless, the damage will be in the range of 152..304, so it is unlikely that you will be able to defeat the Red Dragon.