I have killed these NMs with the time limit, some easier than others, I will say that by pointing out you had Delve & RME DDs you are saying your DDs are good, but were there BRD swaps? Buffs up at all times? Was the NM's weakness being hit at all times to full effect? If yes, then I have not much I can say, if everything is maintained the NM should go down easily with about 5 minutes left, if not, then it makes things much harder.

I say increase it to 30 minutes, I have had many groups do well and fail at under 5%, removing it completely would be bad, but increasing it to 30 minutes, giving a 50% increase, would be quite helpful to those who have groups good enough to do it who fall short because of 1 or 2 mistakes.