Incredibly difficult or impossible, such that it could be seen as a foolish or worthless endeavor to attempt. (Jell-O is a trademark for a brand of gelatin-based desserts that have a soft, gelatinous texture.) Trying to have a reasonable debate with her father is like trying to nail Jell-O to a cross. If they can't secure a victory here today, climbing back to a spot in the tournament finals will be like trying to nail Jell-O to a cross.