This Seeking Tech guide will explain how you can beat Undead Grius in Metaphor.
Make sure you go into the fight with a Healer and Warrior.
At the battle, you will learn about using Synthesis skills. So, make sure you use the Jump Thrust attack with your Knight.
The Warrior should then use the Slicer attack. The Healer should move to the back row and use Hama to weaken the enemy.
Continue using these attacks to defeat Grius.