I dont think many users here have experience with the Korean schools; so my advise is to speak with your letter writers & ask them specific strategies that will fit your situation. You should also look which universities your peers (with similar profiles as you) ended up at. One thing many people can attest here is that, if you have a chance to take a PhD level course in Econ, that will be really helpful! But don’t take it unless you are sure that you can get a 4.0 in it. Hope that helps.