This achievement requires that you kill 3 enemies without touching the ground. You will probably get this without even trying. A good method is to get a "Tough Nut" power-up which gives you invincibility and will let you grapple and swing into spiders or bees. Another good method is to load up the boss level of Year 1. Proceed until you get to the section with a bunch of bees flying in a circle. With some timed jumps, you can kill the first bee, bounce to the second bee and kill it, and then bounce to the third bee and kill it. They are all in a line.