Wait... I might be wrong about this. But Soldier Eddies damage is not consistent enough to use him as "proof".
I have a 5* SE and he does different damage every time against the same enemy. I'm not talking about the damage type, just plain damage. I'm pretty sure there's a damage range but he does not work as, for ex. my corrupted harpy that does the same amount of damage every time (regardless of true damage).
Maybe if we could test this on a true damage dealer we might understand it better.
i don't know
Anyways, I use Streng (Fierce); Guardian (Fierce) and Boost (Will). I sacrificed health and went full attack.