There is a barn in level 2-1 (hug left side of map if having trouble locating) just before the collection of the satellite piece. In here if you walk to the end, they will spawn roughly 140 tiny guys. Any area of effect weapon that can be fired enough and reload enough should be plenty to hit your 100 destruction streak. I did it using the Catalyst, her drone gun and the arc cannon (auto chains to nearby guys).