By using fire AOE and a single staff skill (Blocking stance) it is possible to farm the Gazluk wolf cave (Tower view) with total immunity to monster damage IF you can kill the monsters within 10 seconds.

I suspect that this capability is not a desirable outcome for the game, so will leave it to the devs to comment whether this should be considered an exploit.
Personally, I believe that it is 'legal', but that the mob damage types need to be varied (e.g. add a fire or sonic attack) to prevent it.
It does however, require very good gear, so many players might not be able to do it. If you can 3-shot kill a single mob with AOE spells, then you probably can.

Method:
Identify mobs that only do physical damage (crushing, slashing or piercing). These are typically wolves, wargs, panthers(?), etc.
Equip your fire AOE build with Staff as your secondary (no staff mods required)
Go to your preferred location
Gather a few mobs together
Cast Blocking Stance (you get 10 seconds of immunity to their damage type)
Cast fire AOE (especially Fire Breath) - kill all the mobs within 10 seconds
Loot /skin dead mobs
Rinse and repeat

In Gazluk wolf cave I soloed 6 mobs at once for zero damage (the maximum that I was able to gather for one fight). All mobs died in 2 or 3-shots.