So with all that written and said, here is what I was thinking about


Sakura(6*), , Pink Ladies (6*), Blood Iris (Promo), Edelweis (6*),

For the alst spot I was considering one of these:
Oncidium (Promo) - Onci brings in more skill activation and buffs both Boss Fight and vs Boss dmg which is always great for this team.
Cyclamen Radiant (Promo) - Her whole kit is build around killing bosses. She buffs her team and then she buffs herself. To bad she has AoE and not a single attack type which feels like the biggest downside to her. She has both Anti boss buffs which makes her great. With her own self buffs that weak AoE can actually get to pretty high damage values if you factor in all the additional buffs in the party making it effectively around 4-5x for herself despite being an AoE.
Cherry (6*) - adds more debuff, has a mini AoE vs2 of 3,6 which is pretty good for single target hit as well, buff herself and only 2 more girls but with pretty high % to make up for it and also has both of the buffs so it evens out.
Liverworth (6*) - Liverworth has only Boss fight buff, but its twice the usual of girls who have both for the whole party. She also deals aftershock damage as a little extra and has a 3x1.8 skill which is pretty nice.
Kerria (6*) ... coz, she is Kerria
Trailing Abu Sport (6*) - This girls doesnt seem that strong at first glance, but has a 5.2 ultimate multyplier which makes her into a second Sakura. Also comes with Skilla activation and buffs Skill damage which is also nice. She does pay for it with adding only one type of Anti Boss buff, but it does even out simply by the damage she brings in.

The first 4 girls have pretty massive damage potential in between all of them. There is some skill activation to help out, and a bit of AoE, but mostly the skill set is one which buffs both Boss Fight dmg and against Bosses, meaning that those two stack onto each other, plus your normal damage buffs.
For the last spot I have yet to decide whom to take but there are multiple options to play around with.
What do you guys think about it ?
The fact that one of the most powerful girls in the setup is a Promo girls makes it slightly easier to get the team togather (still not easy I mean at least 3 others are regular 6* so thats the long game setup) plus depending on the pick the last one might also be a promo girl.
Overall as far as killing bosses this setup seems pretty powerful at first glance. There is some limited AoE to deal with trash mobs before getting to the boss.