To get this achievement, you need to be playing on Normal or Perfect difficulty to go into bonus levels. (Normal being easier, as you don't have to grab every gold bar in the Bonus level.)
First, get to level 1-6: Junior Melchkin. Complete the level perfectly, grabbing all gold bars and COREs to get to the bonus level. Above the 32nd gold brick in the bonus level, slightly after a pair of double campfire jumps, there is a spider on the bottom. Rather than sliding under it, jump into it for the achievement.
(If you accidentally slide under this spider by accident, don't worry, there's more in this level and any spider death will do ya, but this is the easiest way to get the achievement to trigger.)