Honestly, if you can't dps well enough to pass T9, you have no business being in Final Coil (and therefore have no need to clear T9), and you should probably spend more time focusing on how to improve your dps. The gate on T9 is not dps, it's learning the mechanics and addressing them. If your dps is too low and you fail even while appropriately addressing mechanics, the problem is a lack of ability to play your role effectively. It's disappointing that SE doesn't give you any in-game tools to check your own performance here (would be helpful to know, for example, that going pure Crit with very little Det is a huge mistake for just about any dps job), but hey, what can you do.