I think you do not only want to get the ticket KUL-SZX, but also the Xiamen Air ticket SZX-SEA refunded as a minimum.
Of course, Air Asia won't do a thing to refund you anything.
You have to try to get the name of the agent who wrongfully denied you boarding. Maybe this chap is still working at KUL.
There are means in Malaysia to collect the money directly from this chap - leaving Air Asia out of the game.
Another option would be to get hold of Tony Fernandes. He is probably quite often into the US. They you may try to catch him with legal means.