Not all impact wrenches do much, most of what you pick up in your local hardware stores won't do anything on a rust seized nut or bolt except make noise. If you can see the threads, you should put some penetrating oil on them.
You mentioned that you see the bolt move, if you're turning it in the clockwise direction, you're tightening it, but you're turning it in the counterclockwise direction and it doesn't come out, you have 2 scenarios (that I can see) - one is that the bolt is stripped, and the other (more likely) is that you have some lock-down nuts or bolts on the tightener assembly. I don't know if this is the case with the Accent, but in general, the lockdown type assemblies have several nuts or bolts that hold the whole assembly tight - the adjustment bolt is only for adjustment and then you tighten the lock down bolts to hold it.
I use the starter motor method myself, never seen it fail. Get your largest socket wrench or breaker bar on the bolt, and wedge the other end against someplace solid on the body, engine, or even the ground. Rehook your battery to turn over the starter. Disconnect your spark plug wires, or unhook the ecu relay. Turn the starter on a little at a time until you hear the bolt "break" loose.