First off, don't use Klook. I used them to buy tickets and they never delivered anything for 2 days.
Secondly, KKday has a discount, which they advertise as 50% but is actually more (I think it is their standard 20% off and then BOGO. I didn't actually do the calculation but it is more than just buying a ticket at normal price and getting one free). You have to go here and register to get the code, and then you go to KKday site to actually buy the ticket. It won't show the discounted price initially but as you go through the checkout process enter the promo code you got before, and viola!
It only applies for destinations Taichung and south.

Great deal!