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 in the JellyFish Fields mission Drain the Lake but just to note this only works if you didn't complete the mission.

This is an easy achievement to get just playing through the game normally, but if you want to get this one quicker, just stop in front of any Duplicate-O-Tron and keep killing the enemies it spawns in. Duplicate-O-Trons are the enemies that don't attack you, but instead spawn in more enemies to fight you. Typically they spawn in very easy enemies to kill. Just be careful you don't accidentally launch an enemy at the duplicator, as that will destroy it.

SpongeBob is the best character when it comes to defeating enemies in quantities. Your basic attack should be enough to kill most enemies early on in the game.