game error at the end of battle just pop up if you guys clear the battle too fast, you can use all mod in once at the battle, dont kill all the enemy, just use defense mode, all your Nikke will deff and no one will shoot the enemy, after that, wait the timer until 40 second, and change to attack mode, and you will clear the stage.
i'm clearing the EX stage at main story 1 with power less then 3000 with this method.
sorry for my bad english.
hope this can help you fam.
cheers