LogFAQs > #901929794

Upon some research, it looks like I wasn't quite right:

https://www.reddit.com/r/FFRecordKeeper/comments/7utpqb/guide_all_about_offensive_boost/

Elemental buffs from Record Materia, Record Dive, Legend Materia, and Legend Dive are their own step.

Elemental buffs from equipment, En-Element, "one of the attack's types matches the type of the current chain", and Magicite in-battle effects are their own step. .

Elemental buffs from "one of the attack's types matches the type of the current chain, which is at X%" are their own step. (Yes, chains give two boosts, one fixed and one scaling.)

Elemental buffs from Magicite passives are also their own step.

So, returning to Beatrix. Since we're talking about RM choice, we can ignore everything except Record Materia, Record Dive, Legend Materia, and Legend Dive.

If you don't have Beatrix dived, she's at 0% for holy and sword, so it doesn't matter.
If you do her Record Dive, she's at 0% holy and 3% sword, so +holy is better.
If you also do her Legend Dive, it looks like she gets 9% holy (19% with LM1) but sword is still 3%, so +sword is better.

...If you don't want to have to bother working that out every time, and you're going to be hitting an elemental weakness constantly, just set Tyro's "+30% damage against weakness" RM. That effect gets applied separately from element/weapon/school boosts, and very few characters get "+dmg against weakness" in their record/legend dives, so it's ideal for most characters - including a fully dived Beatrix.
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I don't like this duchy. Now, it's an adventurer.