Grym Strategy - No Risk, VERY Easy

Before the fight, You can stand above the platform on the stairs and use a mage hand to pull the forge lever (This doesn’t have to be a mage hand, you can sacrifice a party member or use any summon — just make sure you have at least 1 person on the stairs). What this does is puts your party wayyy above Grym. Shoot the lava valve when the platform goes down. When he sees you, he walks into the lava towards you. This puts you in range for the damage source, and keeps him superheated the whole time.

On every turn, throw items at or heavier than 0.5 weight to deal crushing damage (like crossbows, axes, swords, or even books, not items that are normally thrown). Since Grym stays superheated the whole fight, on a hit, it does ~30 or more crushing damage per throw, and you will never get hit. Just have your parry chuck stuff at him until he dies. Make sure you shoot the lava valve again every 2 turns instead of throwing to keep him heated. When the mephits spawn, they are top priority, because Grym can’t hit you. Grym actually tends to fight the mephits, which does the work for you.

This is perfect for solo honor mode (unless you think this is cheating). You never take damage, Grym never heals, and you can easily flee combat if the mephits are a problem. You can also get the metal items you through (though wood will burn in the lava).

Hopefully I explained this well. Happy cheesing!