To beat Grym in Baldur's Gate 3, the key is to use the environment strategically. Grym becomes vulnerable only when standing in lava, which applies a Superheated debuff. The best way to defeat him is by:

• Keeping the lava flowing by frequently activating the Lava Valve.
• Luring Grym to the center of the forge (the Crucible platform) where you can activate the Forge Lever.
• Activating the Forge Lever triggers the Forge Hammer to slam down, dealing massive Bludgeoning damage (Grym's only vulnerability) and knocking him prone.
• Repeat activating the Lava Valve and Forge Lever to quickly bring Grym's health down.
• Use ranged characters to shoot arrows at the Lava Valve and Forge Lever to activate them from a safe distance.
• Be prepared to handle spawned minions after the Hammer hits Grym.

Optional tactics include:

• Using blunt weapons to apply damage when Grym is Superheated.
• Buffing melee fighters with elixirs and weapon oils for better damage and AC.
• On higher difficulties (like Honor Mode), niche tactics such as using a Wild Shaped Druid Owlbear jump attack or applying Silence spells to protect the party may be required.

Following this approach can defeat Grym efficiently, even on harder difficulties, and earns rewards for the Adamantine Forge quest.