How to farm skulls in Orcs Must Die! 2, 15 skulls every 5 minutes! (Only need 2 traps!)

Step 1: Unlock Nightmare mode: you do this by beating the story. The reason for doing this is that nightmare mode reaps more skulls, plus the story unlocks the traps you will need for this guide.
Step 2: Buy the Push Trap: Buy as many of tiered upgrades as the game will allow, then buy the extended range unique upgrade.
Step 3: Buy the Paladin Guardian: Buy as many of the tiered upgrades as the game will allow, then buy the stunning unique upgrade.
Step 4: Play the Nightmare Mode version of the first level titled THE EDGE, selecting only the Push Trap and Paladin Guardian.
Step 5: Place the Push trap along the wall shown, and Paladin Guardians close to the end of the wall in the area where the Push Trap does not extend. Here is a visual example:Step 6: Play through the level, making sure to stand back and place new Paladin Guardians around the same area whenever you can. DO NOT get hit and wait until a wave ends to grab any extra skulls, better to be safe than sorry!
IMPORTANT: REMEMBER TO CLICK CAMPAIGN MENU AFTER THE LEVEL ENDS. PRESSING RETRY WILL RESTART THE LEVEL AND DELETE YOUR HARD EARNED SKULLS!

This section is for people too lazy to read:

Not using mana gives you 6 extra skulls on Nightmare Mode, not being hit once gives you 5 extra skulls on Nightmare Mode, a victory gives you 2 extra skulls on Nightmare Mode and a perfect victory gives you 2 extra skulls on Nightmare mode.