Here is a guide someone wrote up. I've heard people mention the game doesn't do a good job guiding you on what to do so this may help.
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fdocs.google.com%2Fdocument%2Fd%2F17U3FXlx-6uGoS4OYNhkc3sJdlsfvI_hvfFmU8RUg0gA%2Fedit
On the second tab for Defensive the last upgrade says "Elemental Piercing" and the value is a % however the tooltip says "Fixed amount of damage ignored from all enemies."
 "Monies²"){kind=icon}
 "The Perfect Tower")