Kaleid Ruby/Heroine Eli-chan's Adventure
On servants with a Buster NP, are the buffs on Heroine Eli-chan's Adventure and Kaleid Ruby multiplicative just like if the buffs came from skills or are they additive. If they are multiplicative how do they stack up against Heaven's Feel in terms of NP damage?
Different type of buffs multiplied with each other no matter if it is from skills or CEs.
For Eli CE it multiplies up to 44%, slightly higher than normal Heaven's feel. But as a practical approach, NP damage buff is always a limiting factor because there are not many strong NP buffs from skills, while for card bonus we have Merlin 50% and many variants of Mana Bursts. Plus the NP damage bonus from CE is way higher than the other type of buffs (40 to 80%!). So it is generally stronger to use Heaven's Feel or Black Grail if you are concerned with pure np damage.
In short if your servants have access to buster up buffs or is with Merlin, Heavens feel. In fact, with an access to even 10% buster up will make it on par with Eli CE for NP damage.
Edit: suddenly realised my idiotic mistake when re-reading this. The servant need to have access to 26% buster up at least (if there is no other NP up) to justify Heavens feel over Eli CE.