Complete chapter 5 and start chapter 6.
Make your way through The Gapra Whitewood. At the end, you will face the Aster Protoflorian, a Sanctum annihilator class enemy. This boss takes half damage from all elemental magic. It casts various barelement spells to change it's vulnerability. Hit it with the opposite magic to hurt it. If you can stagger it, you can take off nearly a quarter of it's HP. Summoning an Eidolon at this point can be helpful too. Make sure you have plenty of potions, as it is not always convenient to switch to a medic role.
After you beat it, just keep walking to end the chapter.