Comrade Selena seems to forget about the upgraded Psionicists, and if they break through your ranks, you'll regret it. Air magic (speed) + expert tactics + first turn (Phoenixes), and when the Psionicists, Earth Elementals, and Phoenixes start tearing through your army, you'll realize it's no joke. Also, besides vulnerabilities, units have elemental resistances, so magic doesn't guarantee a 100% victory. And Phoenixes can also attack with their breath (through weaker units, targeting stronger ones) with good damage and minimal retaliation.
And this city is perfect against them, so out of these three castles, the Conflux is slightly stronger with roughly equal armies.