Any tip on how to solve hard questions (700+) on quant within 2 mins?

My advice:
1- make sure you can solve easy at 95% accuracy and medium at 85% accuracy (at least)
2- Hard questions are meant to take more than 2 min (till 3.5 min in some cases). so you should be able to solve easy and medium questions at much less than 2 min.
3- Use GMATclub question-bank extensively. Don't worry if you can't solve some of the hard ones. The good news is always that you knew the idea.
4- Use the error log to record the hard ones that took too much time or you got wrong so that you can come back to it later.
5- Many questions in the Question bank are too hard to be GMAT. Avoid questions of unknown sources (usually tagged: please specify).
6- Finish the official guide questions first, then jump to sources such as Veritas - Math revolution - GMATprepnow - Manhattan (I put Manhattan in the last place because I found many questions too tedious or time consuming with less smart ideas)
7- Don't compare yourself with others
8- Focus on practice more than theory. For every question there is lots of shortcuts and theories. Although these theories may save you time, but too much theories would be too confusing and too hard to memorize. GMAT is not testing how much theories do you know.