Thing is, that's the difference between AP scaling and destruction scaling. Let's assume the torpedo is accepted as Low 7-C, this just means the AP of that attack would also match that value based on whom it can harm. For example, Beam took that torpedo to the skull band coughed up blood, making Beam's durability Low 7-C. Denji was able to tank the AP of Reze's explosion in the building. However, separate from Beam his transformation can be worn down much faster while Reze had to pressure Beam significantly more than Denji.
Now here comes the tricky part about powerscaling. In the end these 3 characters still end up being Low 7-C. Why?
If Beam's durability is > Low 7-C, Reze's explosions though to a lesser extent and with numerous blasts, can still wear down and harm Beam. I don't know if y'all have a calc for 8-A+, but if you do we could place her explosions there, otherwise we are downscaling from Low 7-C. Denji's durability also reflects this.
Reze's durability > Beam's durability > Denji's durability > torpedo explosion > numerous explosions