Sandy isn't the best when it comes to crowd control, but her lasso ability will insta-kill any enemy you target it with. Just jump into the air and hit B. This is very useful when facing larger enemies that usually give SpongeBob a harder time.
You can unlock this achievement without much difficulty by just playing through the game normally. I'd recommend stopping in front of a Duplicate-O-Tron if you want to earn it a little faster. These enemies can be found throughout the game, and will spawn in more enemies infinitely. Just be careful that you're not overwhelmed.
Sandy is a playable character in the following levels:
Downtown Bikini Bottom
Rock Bottom
Sand Mountain
Flying Dutchman's Graveyard
SpongeBob's Dream