Full: A fiery phoenix rises at your location, striking adjacent foes for 7...91fire damage, and flies out to your target, exploding on impact. This explosion strikes your target and adjacent foes for an additional 15...75 fire damage.
Concise: Projectile: deals 15...75fire damage to target and foes adjacent to target. Initial effect deals 7...91 fire damage to foes adjacent to you.
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.