That's a fine way to do it.
Senior Centipede is by statement able to take these attacks and since Murata is giving us our only line here, that's what we will have to take. Senior Centipede is 681 kilotons, Pumped-Up Metal Bat can one-shot him, making him low 7-B by default. Only thing is, Pumped Up Metal Bat is far stronger than Garou and would have likely one-shot him if any of his strikes connected. Therefore... I think HH Garou should be: "High 7-C physically, higher with WSRSF", because he has to be blocking Low 7-B attacks here repeatedly, attacks that would knock him out or kill him without WSRSF.
So in other words, he would downscale from Metal Bat to around Senior Centipedes level. Royal Ripper would still be High 7-C I think, Bug God would remain as he is "at least High 7-C, likely higher" | "low 7-B". TTM in that case would also be "at least High 7-C" since Garou had to use WSRSF against him, but TTM a