You want to go through the door on the left and ideally you never position Asker in a way that he'll get hit. Remember to use the "Enemy Range" button and also don't let the boss initiate on Asker
Thank you for your guidance,Maybe I'm really stupid.I tried to keep Asker out of the final duel,but the two mobs around the boss will exchange blood with each other.So there has been no customs clearance.I've tried it, too. The characters all have the same blood volume, Or just let Cervius Suffer from poisoning,But the boss will attack Asker first, which will lead to the failure of customs clearance.