Full: A fiery Phoenix rises at your location and flies out to your target, exploding on impact. This explosion strikes your target and nearby foes for 10...50...60 fire damage. If you are Overcast, allies in the blast radius are healed for 20...68...80 Health.
Concise: Projectile: deals 10...50...60 fire damage to target and nearby foes. Heals allies for 20...68...80 if you are Overcast.
It is best used to target nearby melee opponents to make it more likely that they are hit by both the PBAoE effect (adjacent damage) and the ranged AoE effect (explosion). Targeting lone ranged attackers or casters only make this an inferior and inefficient Fireball.
The secondary effect of this spell is a projectile spell, and is thus blocked by terrain and can be dodged.
If it is certain that melee mobs are targeting you, start casting Phoenix before they reach you. With proper timing it should finish just as they are in adjacent range, delivering a nice damage spike. Follow up with Flame Burst or Inferno for additional damage.
One of the only skills whose description and effect attempt to justify its name: "A fiery phoenix rises at your location...and flies out to your target, exploding on impact." Another notable example would be Belly Smash.