The easiest way, by far, is playing a "Capture the Chao" match with a second controller on the Creepy Castle arena. In the central bridge you'll see a Super Object consisting in 6x Ice Shots. Shoot the second player with them and keep waiting for it to respawn so you can shoot backwars again and again until you get the achievement.
Also, if you make 10 more hits, you'll unlock the achievement for 20 Item Hits in a Single Race