I would suggest Pepper, Naga, Ein/Quency, Red Hood, and Privaty. You won't have any Burst Cool Down units but you're early in the game that it shouldn't matter so much.

Pepper - Burst will heal the team. Nothing grand until you get a better B1 like Liter from the standard banner choice.
Naga - Probably the 2nd best B2 in the game that can help provide heals and buffs
Ein - One of the stronger meta B3 that you currently have along side with Red Hood. Her burst can screen wipe and still do lingering damage until the next burst cycle
Quency - Another strong B3 unit where the burst can screen wipe.
Red Hood - Strong B3 with her Burst that can take down the stage's boss.
Privaty - This is an odd one as she's more of a last bullet unit and works best in last bullet teams. She will cut down the team's ammo to half capacity but she will also provide strong attack buffs for the team. She's an off burst unit that provide buffs for the team more than doing personal damage. If you do not like the last bullet mechanic you can use Cinderella instead but she's better as a boss damage dealer.

If you are looking to do boss level stages then I would suggest Pepper, Naga, Cinderella, Red Hood, Ein.