Out of the three EFBs, Rain is actually the highest buff to base power, since it raises Suizhen 3.5 stages more above Nameless to 18.
(I think Nameless is still at 14.)
Then High Hand of the Norns raises Nameless to 17, and Suizhen to 17.5.
Finally, Tyranshal's Distillation raises Nameless to 16 and Suizhen to 16.75, but also allows Nameless to send out endless Stage 15 armies, though this is probably not relevant because xianxia only cares about top-level battle strength. However, it also quadruples the effects of Potentiation, so now non-direct attack and defense spells get 8 steps bonus. (the direct bonus hardly matters, because I think we only have Mordant Fire as an attack spell, and even boosted 4 stages, it is irrelevant.) Anyway, stack that on Prodigious Might and Quickening, I'm not sure anything else gets the buff.
So, Might gets boosted 8 steps, and at its base boosts 1 stage of physical strength... I have no idea how that works out, but it probably clears the otherwise fairly massive power difference between Distillation and the other options, by raising at least one extra stage of physical strength. Admittedly, a stage of physical strength is not the same as a holistic stage, but eh.
Anyway, I'm not... quite fond of dumping everything on Suizhen and being relatively helpless personally, though Rain makes her cloneable and all. I'm tempted by stereotypical Xianxia Ring of Might, but - stereotypical xianxia.
I suppose Distillation is the only way to go, then?
[X] Tyranshal's Distillation [Blood, Battle]
-[X] Go Even Further Beyond [10 BPs]
[X] Annihilation
E:tfw when I spend too much time rereading and get ninja'ed by Rihaku