The strength of a (Level-100) could be measured by the highest level of creature that they can defeat.
It is a simple logic of
This upgrade which takes the Metabody which one creates at the starting of the game takes the whole experience that one has in the game to the other level, making them more connected with that world, and granting them access to the abilities that they did not know they could use.
The highest level of creature that they could defeat, at this point, becomes the standard of their strength. And, since we aren't talking about simple monsters but also all kinds of creatures present in the game, there are various ways one could access their own unique speciality while defeating these creatures.
-Clank! Clank! Clank!