Wood so you don't damage metal and plastic parts. And you need to press down onto the damper piston to oppose the twist of the piston. The piston is not seated in the bottom of the fork lowers. And the bolt for the bottom of the piston can't be unscrewed because your attempts to loosen and tighten that bolt are just turning the piston.
So you're pushing the piston end down into its press-fit point in the fork lowers while you tighten it into place. Then you may be able to crack that bolt free.