Here's if my guide on how to do the accountant achievement, but let me post a quick and easy walkthrough:
1. Select "Campaign"
2. Select "The Adventure"
3. Go to page 3 and select "A Newfound Friend"
4. Place Da Vinci's Tank
*I also placed some decoy cheap people up front to take some hits while the tank got going, just making sure to spend half, or less than half, the available coins.