Reiner's stats have not been modified. I suspect you have some sort of active debuff.
Viewing post in Was Reiner buffed in between public and latest?
It is entirely possible I've confused routing in my head as a result of experimenting with hard mode at times, and that I usually beat him before demon incarnation on a hard file closer to day 10 after some workshops.
Regardless, by getting to level 3 first with the slime oil quest, I was able to get a lucky run at him where I could live the second rampage because I had dodged both exhausted attacks.
Apologies for the concern.