After arriving in the second major city in Metaphor: ReFantazio, you will be tasked with going to a cavern that has a lot of little surprises. From fighting through the interior of a human worm, to fighting a small bat with shadow clones. This boss fight with the character named Heismay is part of the narrative, and there is only one real quirk to the fight you have to figure out.

How to Beat Heismay in Metaphor: ReFantazio
When you start the fight, Heismay will do his best not to kill the party, so do not worry about getting steamrolled. But he will summon shadow clones to confuse you. But there is one way to identify which one is the real Heismay to ensure you are not wasting turns and most importantly, your time and MP.
If you take too many turns against Heismay in the fight, Gallica will reveal the secret, but if you are stuck at him right now, that is not helpful. So, simply look for the Heismay with the shadow.

In combat you are going to be focused on what abilities to use, or not paying attention to subtle details like this, but yes, it is as easy as looking at the ground. If you use a Brawler Hulkenberg and Swordmaster Strohl against him, 2 uses of the Phantom Swordswarm from Strohl will be enough to beat Heismay and progress the story.
I hope this guide helps you in Metaphor: ReFantazio, if you need any tips to keep in mind while playing the game, you can find a short list here!