>>542583359
100% Isabel + Arcana first. You don't even need much investment on them for them to outperform MPriv by a lot.
A budget investment of 7/7/4 on Arcana, no OLs needed and 4/4/7 on Isabel while focusing on ele dmg and ammo mainly, ATK won't do that much considering Arcana provides over 150% ATK for her already.
I have their skills at what I described and only 60% ele dmg and you can see in the pic she already does over 2B dmg alone. Only my Cinderella outdamages her in the other comps, not even SAnis or CMaiden manage to do over 2B in their own teams with better investment for me.
Ada is fine too since she does good dmg and at least provides buffs to others, she works really well with Ein in a Miranda comp using the 100% crit strat on burst.
Exia is fine too, and probably relatively low investment since you only really need to max her S2 and you don't need or want ammo on her as well as ATK being of less use so you can focus on only getting ele dmg and she'll perform well. Might be difficult to work her into a comp and ideally you should manual her and she works a bit similar to SMilk requiring regular full charge shots while fast shooting most of the time. Leaving her on auto works too.
Trina is actually bait, I wouldn't bother investing unless you have a super strong Scarlet. The problem is that Trina really only buffs S so any other DPS in the team won't get much out of her and running some generalized B2 buffer instead like Grave/Mari/BAde ends up working out better.