I don't think there's a problem with the instance, but I know first time I let myself be caught off guard and didn't properly take care of the adds at the end and was like "oh shi I should deal with that" and didn't do it in time.

Whilst frustrating to redo, I don't like these to just be a cakewalk and for there to be a risk of failure.

I did mine as DNC too. It is a DPS check and arguably DNC has the lowest DPS, so will be slightly harder to do, depending on how efficient your rotation is, as is the case with any DPS checks.

If its still an issue. I'd look at rotation guides for DNC to push your DPS further. No shame in doing it, I ended up doing so for my DNC because I want to push it.

I suppose something you can do (if you don't already) is reserve your Devilment and Technical Step for the DPS check, because the 1500 potency is nice and it comes with an extra DPS boost in the last push with the boosts from both skills.