If you encounter a difficult problem, it's really chewy. A clever strategy to solve a problem is to break it down into a series of steps or small problems. Solve some problems first, solve as many as you can, and write as many steps as you can. Failure does not mean failure. Especially those problems with obvious problem-solving level, or procedural methods, can score every step of calculus. Although the final conclusion has not been reached, the score is over half. It's called "getting points for big questions", which is really a good idea.