The impact is the only way to go when trying to remove the bolt at the bottom of the fork to disassemble. You'll take forever trying it otherwise IF you get it at all.
I agree that the impact is your only hope of getting the bolt out. If this fails you will have to drill the head off and remove the remaining part of the bolt once you have it out of the down tube. I would be careful with heat as the monotube is a sealed system and heat may adversely affect the internals.