I think phoenix is unique in that, with their special ability to survive, I think taking on characters that are a couple gear levels higher than you is totally possible, especially since the characters you'll be facing didn't seem especially strong or synergistic: https://swgoh.gg/db/events/artist-of-war-legendary-event/
So I think (and hope) that gear 8 would be sufficient even for 7*.
Don't bother wasting your resources on gearing Phoenix yet. Wait to see what the general consensus is for gear requirements, then shoot for a little under that, and throw your arena mods on them. No point wasting gear on useless characters.
Replies
5*: Gear 7
6*: Gear 8
7*: Gear 9
This is assuming good (not arena) mods.
I think phoenix is unique in that, with their special ability to survive, I think taking on characters that are a couple gear levels higher than you is totally possible, especially since the characters you'll be facing didn't seem especially strong or synergistic: https://swgoh.gg/db/events/artist-of-war-legendary-event/
So I think (and hope) that gear 8 would be sufficient even for 7*.