The best time to do this achievement is just before the start of Day 3. You will go to bed and be woken up by the Underwear Gnomes. The Underwear Gnomes tend to channel more than any other enemy encountered in the game so far, and so this is a great time to achieve this.
It's recommended to save before you attempt this, just in case. You will fight approximately 9 normal Gnomes before fighting the boss, which should give you the opportunity needed. Channeling enemies will be glowing red and will usually have the words "Channeling" above their heads. You might have to wait for them to channel in order to farm this achievement. In order to not kill the gnomes, select your bow but don't press any buttons when it's time for you to attack -- this allows you to use your turn without hurting them.
To cancel a channel and stun the enemy, use any of your farting abilities. Make sure you have mana potions to restore your mana that will be needed to pull this off. You can do this repeatedly in the same battle if necessary - as long as it's five times.