Enemies won't start channeling until after you finish your first visit to the school. The Boss Hall Monitor will channel at times, as will The Bard. Most notably, the Gnomes channel often. Each time an enemy channels, make sure you have enough Mana (you can use the basic fart spell for 40 Mana to count toward the achievement), then unleash a fart spell on the channeling enemy. It will interrupt their attack, cancelling it essentially, and will gross them out. This needs to be done 5 times cumulatively to unlock the achievement. I recommend making a save and then farming this on the Underpants Gnomes. Fill your Mana before battle (you'll battle 3-4 at once). When they channel, often more than one will channel on the same turn. If you've already maxed your Mana before battle, you can drink a Speed Potion, use Dragon Breath on one of the channeling gnomes, then use it again on one of the others. This will give you two in one turn.
If you don't have this achievement by the time you beat the game, another place to farm this is on Craig in Cylde's Tower. When he makes clones of himself, they will sometimes channel, although not as often as the gnomes, so try to get as many on the gnomes as possible.