I use a team of Dipladenia, Oncidium and Edelweiss.
Using all the tricks for a fast run (low quality, manual control, mouse positioning before the buttons appear, mouse wheel during the fight, clicking the fast forward button off and on after defeating the boss) I'm able to get a ~10s (14s max record) buffer before the 3rd attempt.
The only thing I look out for is when I run out of allies to pick for the run to avoid the "Invite to allies" message at the end.

Thanks for the suggestions. I do all the other stuff (low quality, spam the button, position mouse in advance etc.).

Currently I use

Alpinia (700 Spd, + 150 Squad Spd)
Anthurium (not evolved as only level 32, 700 Spd)
Edelweiss (515 Spd, + 50 Squad Spd)
Setaria (720 Spd)
Gaillardia (560 Spd, + 50 Squad Spd)

My squad speed is 889.

I take out Anthurium or Setaria and my squad speed drops.

Dipladenia is only level 32 at the moment too. I will try go evolve one of Anthurium/Dipladenia for next time!

Well well. Too much girls in your team. If your Alpinia is evolved/max affection and with an enough good level, use only her and Setaria.
If it can help you, I can do the boss in one turn with only Anthurium/Dipladenia max affection, lvl70. Also, prefer allies with high speed team. It will take less time to auto-reach the end after your team reach it first.

Just try. If it's not enough to kill the boss, add Dipladenia or Anthurium. You will have enough speed with them I think. Maybe try Alpinia alone. She is a 6* after all. At high level she can certainly kill the boss alone.

Just remember, the more girls you have, the more attack/buffs/skills animations you have. And all this stuff take too much time.
If it's possible, avoid girls with loooong skill animation like Dipladenia. Oncidium/Setaria are good for that.
In the future, having a lvl1 skill Dipladenia just for that can be a good idea since she don't have any buff at the fight start.

And... that's all I think. Pray for not too long loading and no damn Loading Error.