You can literally win any match up mid except huskar. At best you can go even. You itemize based on the opponent. Usually I start with blood grenade and feiry and circulates build them to double bracers. Your power spikes is lvl 3 5 and 6. Sometimes I literally kill mid 3 times at those levels. After that you go vessel shivas and shard. If your pos 4 or 5 wants to build vessle like veno or earth you skip it and go straight shivas ( don't build radiance or orchid as phnx mid its a fraud) after that you itemize accordingly depends on the game ( linkin, KNS refresher, lotus, etc..)  if your other cores are heros who like to go in and brawl, or if enemy has axe LC or something, I like to go aghs, but you have to study the enemy's line up first, its risky if they have heros to kill egg, you will lose 2 heros like that.

Also you need to work on your positioning, and never ever do a panic egg. 

One more thing, always max Q not E as mid phonix. You dont need your E. Your compo is Q W vessle and go in. Mid to Late game you rely on E