How to get the Secret Shenanigans achievement

As you may or may not know, it's impossible to have every secret unit on any of the normal campaign battles because you need 57,720 gold. What you can do however is have all the secret units on a custom campaign battle (not many achievements in the game can be unlocked this way).

Go to "custom content" from the main menu. Now create a battle where both sides have infinite gold. On one side put down any unit/s you want to fight against. On your side place nothing. Save the battle and go to "campaigns" from custom content and then create a campaign that only has the battle you created with infinite gold. Next you want to save the campaign and then after you've made sure it's saved play it. On your side place every secret unit in the game while the other side has the unit/s that you saved for the enemy. When you've put down every secret unit start the battle. As you will most definitely see you obliterated the opponent and got the achievement. Congratulations you now have one of the hardest to get achievements in TABS.