Be warned: This achievement is very glitchy.
Anyway, play the game until you fight the underpants gnomes on Day 3. The first fight you have with them they will attack you but only do 1 damage. Don't kill them yet. All 4 of them should "channel" at some point during the fight. In the middle of them doing this, make sure you have your mana meter up so you can fart on them. I used the Dragon Shout. I was only able to get three on the first battle. Immediately after this battle ends, you'll fight all 4 of them again except it will actually be a real fight. Again, the gnomes will channel their attacks. Repeat what you did before to get to 5. If you've done this you might be surprised that you didn't get the achievement. This happened to me. All you can do is wait and the achievement should pop. It popped for me about 20 minutes later. I was in the middle of doing the "Just Saying Hi" achievement and as soon as I farted on Kenny, it randomly popped.