I’d usually suggest taking a course
1) Practicing the test format and timing helps a lot
2) If you haven’t been doing some of these math problems in a while, it might help to go over the concepts again
3) Verbal can be tricky
4) The test is adaptive – you should expect to be working at or near the limits of your ability within 10 questions (Caveat for math, where some of you might feel relatively unchallenged the whole time)
5) Multiple attempts are not viewed positively if you decide to try again
6) Higher scores might help with scholarships
7) Courses are relatively cheap

However, NUS GMAT reqs are relatively low at 662 for full-time students. For a scholarship, you would need at least a 700, which I would expect to be relatively common round these parts, and your deadline is a bit too close
http://bba.nus.edu/bba/concurrent_faq(Oct2012).htm

Anecdotal source: Got a 770, and this is the same advice I got from the other person I know who got a 770

Anecdotal NB: From what I hear, smart Singaporeans tend to do really well on Quant and sometimes not so well on Verbal.