How to Complete A Lady's Insistence
The objective is to weaken the Nagaraja, NOT to KO it. If you do KO it, you will fail the quest. You just need to bring the Nagaraja's HP down until it is HP Critical. You also need to make sure that Syrenead does not get KOed.
Attack is forbidden in this battle. This just means that you are not allowed to use the Attack command. However, you can use job-specific attack moves to deal damage.