Disney Infinity

One way I found that was easy to get this achievement with is in the Monsters University hub. When you get the Paintball Gun after returning from Fear Tech, you'll unlock a Paintball arena. Go in it to find four challenges, three of which involve shooting opponents with your Paintball Gun. Go through these three challenges on every difficulty and if you don't unlock it by the time you're done with all three difficulty levels, you'll be extremely close. Head into the Toy Box and thrash a few enemies until it unlocks.

Another way to get it is to do the Gladiator adventure, which is unlocked after completing the Mastery Adventures in Toy Box. You'll be pitted against numerous enemies in order to complete it. Just as an example, I killed 67 enemies before getting to Silver and that was a practice run.

Good luck!

Just play the game, or go in to the Toy Box and place enemies to kill, running over them with a vehicle speeds things up fast.