Since Ryota Iwagami asked for which monsters the players are talking about, I'm going to list the ones I've fought.

Perdurable Raptor - This one is considered the easiest. Which it is. Assuming you have all acc songs/rolls/food/JAs up. And assuming you don't get zombie PLDs while its enraged.

Mastop - this one is TERRIBLE. You cannot debuff it (except with GEO) and it's evasive as hell already from being a gnat; not even counting the fact it's a delve NM. My group did SMNx3 timed Odin each doing 10k Zantensuken at the start. Only took it down to 95%. Fought it raged well enough but then random back-to-back triple attacks were killing our PLDs.

If there is one thing I DO like about these NMs is the loot system for the armor/weapons. Beating them once, then being able to buy the gear from an NPC with points? That's the best loot system in this game so far. Keep that. Change NOTHING about it. Ever. >:[