You have to fly THAI. THAI are currently offering a +25% deal, over and above the usual class of service bonus. It is all available on the THAI website, which I commend to you to read:
http://www.thaiair.com/frequent-flye...on-THAI-EN.pdf
Can't fly First class SIN-BKK-SIN.
Finally, I think you might do some more research yourself and ask more focused questions.......