This is obtainable in multiple places but the easiest is as follows.
When you are shrunk by the 4 underpants gnomes at the beginning of day 3, you need to interrupt their channel attacks with any of the farts, (easiest being dragonshout). You may be lucky and interrupt all 4 in that battle but if not, another 4 are at the end of that particular section.
As long as you interrupt 2 in one battle and 3 in another, this achievement will pop once the 5th has been interrupted.
Other enemies to interrupt are the ginger kids during the detention mission and the clones during one of the final battles.