There are a lot of tough bosses in Metaphor: ReFantazio, but Grotesque Guptauros is one you will face fairly early on in the game. You will find Grotesque Guptauros in Grand Trad, which is the main city of Euchronia. There is a side quest called A Bullish Embargo tied to another dungeon, which you can get from Brigitta, the Igniter Shop owner. This dungeon will eventually lead you to Grotesque Guptauros, which is not an easy fight if you are not prepared.

https://screenrant.com/metaphor-refantazio-how-to-beat-grotesque-guptauros-boss-guide/