One of the biggest recommendations I can make for new players is to reroll characters until you get ones that don't suck, lol. Just because the girls will always cover for eachothers minor/core vulnerabilities, doesn't mean they're all that viable to play as. If you're going to spend hours playing, might as well make sure it's a team that can clear the game. Also don't be shy to beat on a girl's minor vulns a bit, especially at the start. Trying to avoid them completely will slow you down too much.
I used to run whatever and always had trouble, now I restart the campaign until I get a girl that already has her minor tier 2 vulns broken and that takes enough circumstance damage from good multipliers (Anything but Exo, really. Exo can come from a secondary character.). Already having her minors broken means you can use anything to build up her circumstance damage quickly. Even though her minors are already broken to tier 2, she won't be friendly with the girl who would normally be friendly with her for breaking them, makes shit so much easier because breaking her minors any further on accident is extremely difficult. You can easily get her to break all her other tier 2 vulns and up to +5 EE at end turn from unresolved traumas. At that point, it's safe to start working on the other girls because she'll already be rivals with at least one of the girls (Possibly 2 if she had 2 core vulns) and you'll have good EE income to upgrade a shit ton.
With a girl like that, it's pretty easy to get the special event bonuses at turn 15/30/45, which is even more EE. That's a huge reason why you can't afford to use any team or focus on any girl, you really want to hit those special landmarks in time for the bonus EE. Basically there are time constraints beyond just the end of the game if you want to be able to upgrade shit in time.
You can already get a good idea of what the third girl will be like from comparing the initial two, which may save time:
Shit gets messy eventually once all the girls have tier 3s broken because they start using OP moves in battle that can reduce surround turns, greatly limit damage allies take, break out of surround, etc. Unlocking the final vulns all at once is a nightmare and a lot of grinding near the end. Can't help much there, it's mostly a waiting game. To break the final vulns you have to let the girls use their special moves a shit ton unti lthey reach 1000%. Can't say I enjoy the tier 4 vulns.