SpongeBob SquarePants: Battle for Bikini Bottom - Rehydrated

You should be able to get this in normal play but if you want to speed things up you can quickly grind out 100 robot defeats when you first play as Sandy in Downtown Bikini Bottom find a Duplicatotron and keep destroying robots until the achievement unlocks.

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