Credit to AceCardJones from TrueAchievements:
I found this achievement easiest to get in the first Battle Race you come across in World Tour on C class, I believe it's the Roulette Road stage of the second World Tour (Frozen Valley). The trick is to hang back in last place for the entire race picking them off as the opportunity arises with triple ice shots or fireworks, even firing your blow-fishes forward works well.
You're safest at the back and you just have to watch out for them trying to lob things back at you, but they're far more aggressive when you're ahead. You have 3 laps to hang back and pick them off so you have plenty of time. If you get hit at all just restart. Achievement pops once finished.