[@Vocab] Ludmilla sounds like your best bet thanks to her lasers and ability to defend herself while camping with her Dorito shields, though you'd have to ask Bad Juju about specifics like range. Frederica has cannons but she's much more of a berserker drain tank. Kei is spec'd for staying safely distanced while focusing down a single target, but she does that more by being a slippery, mobile skirmisher than by avoiding notice. She wants to be seen. A viable option might be to steal a rifle for Monika, for obvious reasons. She's a little under-supplied and can hide it in her sash. You may need to teach her to shoot, though.

I wrote up a whole list of everyone's characters and a loose description of their combat class, role in teamfighting and specialties, but it's saved somewhere and I don't think we need it.