Dont you have an ak 47 and spas shotgun at the end tho? Also what difficulty are you even on? I always play on the hardest and like i said, aside from act 3 train fight, its easy to win.
If you dont like kihara(minigun girl) simply swap her with another girl, Aya will then take the guard duty with ya. Also dont heal mid fight cause thats a nice way to not only lose your health but get your other girls swarmed, fight or die. If they are getting negative, melee hit the enemy and it stops them. If this doesnt work, remove the game and save file and reinstall it, cause something wrong is afoot here.