700-720 is a very difficult goal. Keep in mind that most people never get such a high score--that's what makes it desirable!

You could take a course, but you might be bored with the quant part! You might do well with our GMAT Interact program--it's a complete online course, but you can skip and work on the parts that interest you. One way or another, you need to make changes in your approach to see any significant improvement. Work through a curriculum--any curriculum--and make sure you are open to truly changing how you read and respond to the material. Doing a bunch of problems only helps when you have a strong technique and you can review those problems to fine-tune your application of that technique.
Best of luck!