This is pretty easy to get if you repeatedly play stage 8 since you can boost through the majority of the level getting at least 100 kills each time. Yes that means you'd have to do it at least 100 times but assuming this is one of the last achievements you pick up after beating the game and completing several of the challenges, collectibles, etc. You probably won't have to do it more than 20. It's still kinda ridiculous though
But you might get this while 100%ing the game