After the very first Tower where the 15F boss Trigger wiped my entire party (well, almost), I'm much more scared of 15F than 14F. If I had two Guards, yes, I would. But I only have one Guard for Fire, and as such, I'm bringing it to 15F.
How nice would it be to have SR Nergal...I actually didn't know that, no.
Still, Kishar will have -50% damage reduction up, and thus hopefully eat at least two, if not three hits.