If it's a decent width, you can rip it into 2-3 pieces then flatten each piece and then reglue it back into one slab.
That way, each of the individual pieces will have less material needing to be removed.
I would also recommend restacking and letting it rest a bit because once you remove the material, you may have exposed some higher moisture wood from the inside and will try to twist some more. You can sticker and dry the individual pieces or the reglued slab but make sure you put plenty of weight on top. Some species really do need as much weight on top as you can muster.
That way, each of the individual pieces will have less material needing to be removed.
I would also recommend restacking and letting it rest a bit because once you remove the material, you may have exposed some higher moisture wood from the inside and will try to twist some more. You can sticker and dry the individual pieces or the reglued slab but make sure you put plenty of weight on top. Some species really do need as much weight on top as you can muster.