Phoenix is a ranged strength support hero capable of dealing large amounts of AoE magic damage, often at the cost of his own health.
Unlike most supports Phoenix does not have a stun or any form of lockdown, but he does have a very strong attack speed slow.
Skills
Skill Icarus Dive first in case you need to escape/chase enemies, but max it last.
Skill Fire Spirits next and max it first. Note that the attack speed slow is incredible; make sure you land it on carries.
Skill Sun Ray at level 3 and then max it second. Remember it can heal allies as well as deal pure damage.
Always level your ultimate Supernova (levels 6, 11 and 16). Use for clutch escapes from certain death or in teamfights as a distraction and potent damage dealer.
Items
Starting Items: Courier, Observer Wards, a set of Tangos, a Healing Salve, 1x Iron Branch.
Early Game: Flying Courier, Observer Wards, Boots of Speed, Magic Wand, Bracer.
Mid Game: Mechanism, Power Treads of Tranquil Boots.
Luxury Items: Drums of Endurance, Shiva’s Guard, Rod of Atos, Heaven’s Halberd, Force Staff, Veil of Discord, Heart of Tarrasque.
Situational: Snetry Wards, Dust, Gem (vision), Necronomicon (push strat), Urn of Shadows (early healing), Pipe of Insight (if someone else is going Mekanism), Ghost Scepter (physical immunity).