This one is pretty easy to cheese. Make sure you've levelled up all your characters to lvl 20 and then buy a lot of healing items and guns. Give the guns to Kiryu and Saejima and make sure every character has their inventories full of healing items as well.
Then head to the fight and during each fight lose twice on purpose so you can set the difficulty to easy. You can use the guns to beat the harder Kiryu and Saejima fights and the other fights you have to beat normally, but that shouldn't be a problem on easy.