Gawain's Artifice are her burns, which makes your local galactic neighbourhood star look like a brown dwarf in comparison. Your team will quickly turn into a pile of stage girl-shaped cinders if you don't do anything about it.
Gawain is immune to MaxHP% DoT.
Anyone that can cleanse or block burn is useful, except Benzaiten. (why would you bring a Moon tank to a Cloud boss?) The main counter to her Artifice is Merlin Nana, who has a passive burn block to party and a 2AP which does the same.
Either that, or use a copious amount of heals, as we shall soon see.
1) Merlin Nana
You must have spooked her, I guess. Congratulations, you have absolved yourself of the need to overwork Shiori.
Any Space 4★ team with one tank and sufficiently ranked attackers will do the trick.
2) Midshit sets records (again)
Too much high US girls and no time? Try this out for starters. 3 turns - focus on Caesar's acts and Tsukasa's buffs.
1) Cheerleader Yachiyo Solo
By correctly casting Exit Evasion, Yachiyo can continually keep herself alive and recast exit evasion. Use the HP regeneration as necessary to ensure you don't die in between exit evades, but balance that with damage to avoid crossing turn limit.
2) Cheerleader Yachiyo + Merlin Nana
Use exit evasion to keep herself and Merlin alive and repeatedly cast CX.
1) How is Shiori not dizzy?
Notice that Shiori does a full 360-degree twirl every time she hits. That's some eighty-plus twirls in the span of eight and a half minutes. Let her rest, okay?
In all seriousness, this team uses one burn cleanser (for the inital 99t burn), Awakened Shiori with a defensive memoir, and high defensive Unit Skills of either the midrow or space variety. If you include an offensive Unit Skill of the midrow kind at a high level, you could consider auto. 4★ Siegfeld 1st Anniversary memoir is also helpful.
While Shiori is not in danger of dying even on auto, she has a penchant for not attacking four times a turn, which lowers your damage output enough such that you might lose by turn limit.
A revue song that increases Siegfeld mid's ACT or damage to space will help.