I am going to go ahead and disagree with the guy above. Grinding to beat this mission won't help in the long run.
Noone can give you any advice on how to beat these battles because noone knows what your advancement has been and what you have access to. If you made poor decisions because you didn't get the obtuse underlying mechanics then you may very well have just scuttled your game entirely.
This game is fucking awful on every possible point. The more you grind, the more classes you unlock, the more levels you gain, the more you help your enemies. The enemies you face will receive every benefit you do plus weapons a tier above your own, stat increases you can't get, and always with optimum builds while you are stuck making shitty class combinations in order to further your own advancement. The minute you unlock a class you will begin fighting enemies with that class already maxed. You can never arrive at anything like parity after the early game. You will always be placed in a disadvantaged position, you will always have less movement, you will always be weaker than your opponents, you will always be more poorly outfitted, and playing with less synergy. Just wait til you get to midgame and suddenly your attacks start missing for no reason despite having attack as maxed as can be with a passive skill bolstering it even more. You'll rarely see enemies miss though no matter how high your defenses are.
Bring a heavy magic group? Half your opponents will have the passive to negate all magic. Bring a combat heavy group? They will have passives that debuff so after every attack you make you have to spend a turn getting rid of the negatives allowing them two turns to your every one. But don't bring too many support units or else the game will punish you with combat that goes on FOR-F***ING-EVER.
And the enemies always know what passives you have and act accordingly while you have to figure things out blindly for yourself. Take the No Flank passive on your entire squad, you will never see an enemy try and flank again. They always know what the max movement is of your entire party so when you do start to dominate they can cheese away to little corners where you have to run them down.
You would think this makes the game difficult but it doesn't. It isn't hard, it is just tedious and absolutely frustrating. It is a poorly conceived, poorly designed, and poorly executed game.
The only way to win is not to play.
