You send in one team to use all the specials from the enemy team. This sacrifice team doesn't even have to be good. Then you send in your real team.
Gal war is the first full roster part of the game you get to. It's also the best one until it gets too easy. You need to learn to use your roster.
Use your best team in the first easiest node. Make sure they end the battle with all abilities test to use. They are now turn meter loaded. That is, they start with more than zero turn meter, except for whoever did the last move. That will let them best all kinds of teams they couldn't otherwise beat.
From the second node in use your second best team until they fail. Use specific counters if they come up, and you have them. Like if Phoenix are giving you trouble take something that has daze/stun to stop counters and healing immunity to stop kanan's heal.
Use your best team as the pinch hitters to get you through trouble.
You can retreat and try again with no penalty. The rng seed starts the same each time, so don't just do the exact same thing. If they get a lucky dodge or resist that undoes everything, restart and do something different that uses up the rng list in a different way. Say you did hera basic on someone, then try and land staggers with sabine, and the important enemy resisted the stagger. Next attempt use hera to call kanan instead. Kanan will use up 2 rng numbers off the list, as he does damage, and tries to apply a debuff. Now you are at a different point in the rng list when sabine rolls her minibombs out. Different outcome.