To those who are lagging on the later stages:
1. You can right click the black bar at the very top of the game screen and select Quality => Low.
2. Try getting rid of some of your towers by getting the "Temple of the Monkey God" upgrade. This reduces lag MASSIVELY.
Hope those two tips work for you =)
How to get 15000 + Pain points and unlock "Chinese Water Torture":
1. Use the adrealine and boost your health to around 190
2. Reduce the temperature to 0
3.Compress, click bees and reduce temperature at the same time
4. Right before your health hits 0, press scarab