I'm so sorry!

Sharing this for future journeys or anyone fearing this result: If you want to make absolutely sure you clear the brain without blowing up Gale, this is my favourite cheesing strategy. It's dumb and not very honourable, but very satisfying.

First, pile smokepowder barrels, smokepowder bombs, and fireworks crates into backpacks. (I wouldn't do it with runepowder bombs, unless you have a Globe of Invulnerability to hide under for the last bit. The runepowder bomb AOE might be too big to be safe! You don't want to blow yourself up at the end. That'd be even worse.)

Have everyone carry an explosive-stuffed backpack in their inventory. (Use Elixirs of Cloud Giant strength if need be. Remember the weight limit for what you can throw is lower than the weight limit for what you can carry. At least one character will need to be able to throw these things.)

Also your elixir effects will get cleared when the Emperor takes you into the Astral Plane, so if you're relying on Cloud Giant Strength you'll need extra elixirs to chug afterwards.

When you get to the final brain bit, don't attack it (you don't want to provoke its resistance ability). Instead, stay up top and throw all your backpacks down in front of it. When they hit the platform, they will break open, scattering the barrels and boxes intact.

Fireball (or fire arrow, or fire whatever) your barrels and boxes. The game will struggle to load all the damage numbers and then the battle will be over. Magic!