I did some research about MRs (Memory Receptacles), and i found they have different elemental resistance depending on which promy you fight them. Is easy to find to what element they are strong only checking the color of the promyvion you're doing.
Example: Promyvion-Dem is green (and MRs have the same color), and it means MRs here are Wind based. This works with other promys too.
So, i discovered to what element are weak all the MRs...
·Promyvion-Dem MRs : Wind based, Weak to Ice, Resist Wind and Earth.
·Promyvion-Holla MRs: Ice based, Weak to Fire, Resist Ice and Wind.
·Promyvion-Mea MRs: Water based, Weak to Thunder, Resist Water and Fire.
·Promyvion-Vhzal MRs: Dark based, Weak to Light, Resist Dark. (i'm not sure about this one).
With this knowledge, is easy kill those things without a +4 ppl party if you have a SMN with the avatars needed in your pt (with my NPC fellow i can solo MRs in the first floor as SMN using a Yagudo Drink without Astral Flow).
Btw, this works too with Mage Boss in every promy, Dem NM is wind based, Holla NM is ice based and Mea NM is water based (in promyvion Vhzal you fight the other 3 promyvion NMs at the same time).