An easy point to start breaking vulnerabilities when you're not that fit in the games mechanics is when you have a 2 times 4 turn capture commander. Try to go on a target that is weak to INJU (or at least not strong agaist it)
Start with your primary target try to get as many circumstances to lvl 1 starting with INJU and then following depending on resistances.
Use the second capture on another heroine where you can put EXPO far into lvl 1. You won't get to lvl 2 (10k) at this time but she shouldn't be able to regenerate out of lvl 1 in a few turns. You usually still start with pummel to push INJU first.
After that try to get a surround on your primary target before the extermination reaches 100% so you can get another one in again later. This should allow you to get some vulnerabilities broken.
The specifics depend on the team but that's the basic outline. Don't be afraid to just save and try the day over and experiment. You can go earlier (as you can read
here) but it's obviously more difficult to achieve.