Thankfully, this is a fairly simple, though time consuming process. Many people try to grind out through Faces of Killed 3, but unless you have a script and time to leave it on for hours, this is not optimal.

• Load up Slayaway Camp 1-8: Tiki Trickery
  
• Finish the level. Solution: ↑ → ↑ ← ↑ ← ↓ → ↑ ← ↓ ↑ → ↓ → ↑
  
• Clear the coin challenge

This will net you a solid 100 coins every time you complete it, about 100 coins every minute or so, depending how fast you clean through it.

No luck needed, and get to buying!