What I did was Combining this Achievement with the Achievement for Completing a Level without Dying
So I completed Level 2 "A Look inside" without Dying and after that Level was done I went back to the Main menu and started the Level "Things you wouldnt Believe" and killed all the crawling Spiders at the beginning of the Level and Immediately after ive killed them all the Achievement Popped.
That way you can do both Achievements together :)
If you prefer a Video Guide I have uploaded a Video of me doing exactly this Method
Good Luck on obtaining it Guys