Due to the dragoon's damage penalty against smaller units, Fenix isn't as effective at fighting the zerg as in his zealot form, but he makes up for it with his ranged attack and ability to attack air units. Fenix is very effective at fighting the enemy protoss in "Homeland" and "The Trial of Tassadar," able to destroy enemy dragoons and scouts quickly while taking little damage in return. Later, in "The Kel-Morian Combine" he is effective against the terrans.
In "True Colors," Fenix is an NPC "boss" hero. He has increased shields and HP of 800 each, and a stronger attack that deals 60 damage, as well as five armor. A swarm of zerglings can kill Fenix easily enough, but the easiest way is to use the queen's Spawn Broodling ability to kill him instantly.