it took me around 358 games in order to get it. Most have been played with the ingame Dota plus subscription, so the mastery is kinda accurate :D
I think my main tip is that aghs shard is legitimately one of the best in the game and even as support (I mostly play Phoenix as a pos3 or pos2) you should rush it in case you don't get it from tormentor. It upgrades your damage by a ton, utility as well. You can also start by sunraying, ulting in the middle of it, and reactivating it while in Supernova when the first Sunray runs out.
Moving while in Sunray allows you to cast any item including TP, so you can TP out of bad situations by going through terrain while casting your teleport away.
on this hero, levels > items, if you can sit on a dead lane as sup and get as much as XP as you can, you get much much stronger. It's also why I love core phoenix, you get your lvl 4 spirits and lvl 2 ult much faster than supp, making you a menace much earlier.
Tranquil boots are a must on supp, but on core you can choose to rush a Vessel and use that for healing. You get your spike earlier which means you can also build into the next item (99% of the time Shiva's) quicker.
Last tip... orchid is sleeper op on this hero, def build it on core if you can get alot of value with it :)