Phoenix

Tips

 * Since most of Phoenix's abilities cost health to activate, health regeneration and strength boosting items should be a top priority. Urn of Shadows, Mekansm, Pipe of Insight and Heart of Tarrasque are good examples of such items.
 * As such, health regeneration items are a better choice than those that simply increase health, like Vitality Booster and Rod of Atos, as simply increasing health will make your Icarus Dives and Sun Beams much harder to maintain.
 * Since the only thing that can kill the Supernova are auto-attacks from Heroes, it's best to prioritize a Shiva's Guard as your first or second major purchase, as its passive attack slow works during your ultimate.With a bit of timing, you can also use Shiva's while Icarus Diving for a long ranged AOE disable.
 * Buying Eul's Scepter of Divinity can sometimes be a good investment, as you can still Sun Ray while under the effects of Eul's, and you can use it while diving to disable an enemy from a long distance. Do note, however, that the mana regeneration bonus is almost wasted unless you plan on spamming fire spirits and sub beam, as Phoenix relies more on health rather than mana.
 * Fire Spirit's attack speed reduction is great to mess up the enemies' last hits. Also, if used right before Supernova, it will make killing the sun harder.
 * Phoenix can be used both as a semi-carry and a support. If going for the former, take the mid lane and buy items to make you harder to kill. If you're supporting, consider going for cheap healing items such as Tranquil Boots, Urn of Shadows and a Mekansm.
 * Radiance is an excellent item to get on Phoenix if you are focusing on using Icarus Dive as well as Supernova after an ally initiates to increase the maximum damage you deal in team fights.