So got this thing of Uptala the other day. It worth using outside of random situational use of the no-weapon-type-on-WS effect?
FAQ and Summary
Murasamemaru hidden effect is that its weaponskills deals Formless Damage. While some may argue that Formless Damage is actually "Non Elemental Magic Damage", many situations in this game refute this idea and define it as a damage class of its own (see #9). Generally, the uses of this Great Katana is limited to enemies with extremely high Physical Damage Taken Reduction. (When Magic Damage would typically deal more).
Here is a list that I have compiled that specifies these situations.
1. Cardamon Custard (Nyzul Unchartered)
2. Pil/Caturae Shields (VWNM/Prov)(Swap in with Dusty Wings)
3. Elementals (Specifically: Walk of Echos Caturae Chamber)
4. Turtle Type Mobs that don't turtle* (Legion)
5. Raska Stance Mode -50% PDT(Uptala/Hahava/Legion/WOE)
6. Arch Omega/Arch Ultima. Omega: Full damage on 4 Legs mode. Ultima: Full damage on Physical Shield mode.
7. Armed Gears - Takes full damage the entire time.(ZNM)
8. Cerberus + Cerberus Seether - (Salvage II- Bhalfau) (Recommend to use Murasamemaru after reaching 50-60%)
9. Sandworm Absorb mode - (Legion) Mura will deal full weaponskill damage without healing it despite the mob being in Physical or Magical Absorb mode. (Interestingly, it works for the zone boss for Abyssea- Grauberg also)
Commonly asked questions that are tested:
Q. Does using Murasamemaru make ranged weaponskills such as Namas Arrow Formless?
A. No, only the g.kat has the "formless" feature.
Q. Does this work on mobs such as KS99 turtle where it goes into its shell?
A. No, it turns out that anything like the KS99 turtle (turtle mode) as well as Urganite turtle modes such as Melomelo and bibikibay are entirely immune to the effects of Murasamemaru while in their shells.
Q. Is attack/defense taken into consideration for "formless damage"?
A. Yes. If your attack/str is low, your damage will be low. The formless damage only applies to the last -pd part of equations. (Elementals have -75% physical damage taken, so murasamemaru will do 3x damage of a normal weapon of the same damage rating).
(Q.)Are the melee hits also Formless?-
(A.)No! Only the weaponskills will become formless.
(Q)"At what point will the advantages of an Empyrean/Relic/Mythic weapon outweigh the benefits of formless WS?"
~25% DT. Any DT higher than that, Murasamemaru will win. Any lower, the advantages is obscured versus any of the other 3 SAM end game options.
If there are any other mobs that you'd like me to test, feel free to let me know.
Edited Feb 21: Added more mobs that it is effective against.