Himself
It all seemed pretty hopeless for Tony in the first trailer we got, but the TV spot released at the Superbowl changed that. In the short teaser, we see a shot of him and Nebula doing some kind of engineering similar to when he escaped that cave all those years ago.
Back then, he built a suit himself to escape, could he do so again? I mean he is genius, and he has a high-tech cyborg on-board with him, so a combination of his mind and her tech could perhaps give them a way to escape.