Currently, ABF characters are either: 9-B, High 8-C, Low 7-B and 2-C (the Noth Potestas has a 7-B rating, but I believe its a mistake as the one that performed such a feat is just straight up 2-C); no Rimuru's key seems to fit with does ratings (unless you want to use a Low 7-B such the Etrien and Agni, that may result in an AP stomp).