EDIT:
Here's something interesting for you Ninja's out there. I've been following this guys blog for a while now and I've read through pretty much everything (with shitty google translator lmao).
http://ku-so.hatenadiary.jp/entry/2015/01/02/122726
If you scroll down, he talks about Fuma vs. Ration. His conclusion is this, if I've read it correctly:
If there's no vulnerability debuff (TA), use Ration
If you've just used TA or there's a vulnerability debuff, use Fuma
If there's Foes Requiem, use Ration.
Due to how shit google translate is, I can't tell whether or not he said to use Fuma or Ration with TA + Foes, I'm leaning towards the former. So yah, Firana is correct in a way with his Fuma Opener.