A lighter won't do it, a mini torch would be better. Have one of those torch lighters like people use for cigars? The idea is just to get that spot hot and see if it lets go. Don't do that yet.
I assume you've already tried penetrating oil? First priority is not to damage what you've got.
I'd think about possible reasons it's stuck in the first place, like reverse thread, rust, super glue, fastener is peened on, locking pin through the nut (never seen that, but the point is to look carefully without preconceptions) . . . take a really good look at it and think about it before you make your assault. It's often the case that in hindsight there were signs overlooked. Judging by the scratches, is the nut spinning around but not coming loose? There must be a way to access the head of the bolt, or whatever that is. This was assembled, you need to understand how.
