As with comics, her powers aren't perfectly consist. For the most part, the stronger the force inside her barrier, the more strain it puts on her. She can form the barrier into crude shapes, mostly a bubble though when it comes to trapping. It has also taken the form of a flat wall, a dome, that sort of stuff.
Without Human Torch, this fight can go on forever. Reed Richards has always incapacitated The Thing pretty easily on many occasions with his stretchy powers, so I see Luffy doing the same. In fact I am not sure how relevant Thing is here at all when it comes to hand to hand combat.
The only option I see as a betting man is Reed being a super genius and figuring something out. Other than that, it could just be he and Luffy punching each other into eternity. I am however unfamiliar with the later years of Luffy, and how much power he gained.