I'm revisiting a game of my childhood, to find as many feats as I can: the game (or better, games, since it's a franchise of around 10ish games or so) is called "Invizimals" if you want to know.
While unsurprisingly it hasn't many feats, I found a few interesting things that could help:
An Invizimal called "Erebus" (a giant igloo turtle that lives in Antartica)is said to be as big as a mountain multiple times, even in the trivia and apparently the creature even is often mistaken for a mountain when he doesn't move. While I don't know if this applies to all of his forms or just to his adult stage, this should scale to all the "regular" Invizimals.
From what I heard is "as big as a mountain" grants you a 7-C tiering via sheer size alone. Is this true?
Another thing that I found to be a possible scaling point is a Goddess Invizimal, called "Pahoehoe" (based on the actual mythological being) that resides inside of the Kilimangiaro mountain, and apparently controls all volcanoes in the world.
While it may be a bit vague to give her a tiering based on this, it would at least give a planetary range, and she can make volcanoes erupt.
My question is, how much energy is it needed to make all volcanoes in the world erupt (at least, the currently active ones, but she has been around for thousands of years, so...)
Again, maybe not the best thing to do, but it's at least something.