My guess is you either:

1. Aren't playing your job properly. As in you're not doing what is necessary to output the requires damage.
2. Don't have the proper items. As in, you are undergeared. My guess is that this is the problem, so if your ilvl is way below 340, you might wanna try to upgrade.

Another thing that comes to mind is how you go about doing the fight. I remember when I did it, I just basically tried to kill all the things. But towards the end, I realized I was only supposed to kill one of the robots while Yugiri took care of the other.