If what Phainon does needs to be calculated, it's not going to be 3-B given it wasn't calculated through him destroying galaxies but rather through his large size (I don't know what method should I use than this, because obviously atomization and subatomic destruction can't be used and like the result is too underwhelmingly low, if like you used the melting point for the incineration part then again 95% of galaxy's matter can't be calculated due to it being dark matter anyway).
I haven't calculated him through large size calculations yet, but I'm sure this warrants like 3-A or higher into 3-A. There's no way this is 3-B, I mean just look at the page
In fiction, there are many characters of Large Size. When it comes to tiering them their size alone can be an indicator of their statistics. This page explains which statistics can and can't be calculated from size alone. The physical attack potency, and with it the durability, can be estimated...
vsbattles.fandom.com
Phainon's going to inflate it (If you don't know it yet why it's going to be inflated, it's the sprinting speed part of Phainon) but again I haven't calculated it so