Much like Aglaea on release, at E0, Phainon is specifically a MoC merchant, specialized in dealing with bosses whose gimmick is pure dmg and survivability check. He is a character with 400 spd that deals dmg in his turn, so very very high floor. But his performance is extremely unstable. Any bosses with a stalling mechanic is his worst nightmare. If you can't have meaningful progress within the first 2 ults, you're very much screwed. PF, 40k is not too hard, not too easy. AS, unless you can brute force phase 2, he'll forever stuck at around 180x (if the buff is full ulti refund) or less.
Now then, how do you construct the best team for him? After many testing, a support that can give him rare buff (vul, true dmg, res pen, def shred) will be much better even if they can't give him
ST buff, or they can do both and be the goat like RMC. The 2nd slot, if you don't have limited eidolon on support, is always gonna be e2 bronya. She enables atk boots on him while still allow for spd sync after ult, as well as provide a perfect checkpoint to end wave 1, useful for all kinds of players. Her downside of buff uptime is completely irrelevant with him. The last slot is a flex until we get his BiS cough
Also, according to HoS, Sunday is "not important at all" to Phainon and can be replaced with Tingyun or RMC and "should get his LC instead if you're rolling for sunday". Don't ask me if that's true.