Another easy place that I found was located in mission 3-6. After the second checkpoint is reached there's a red button upon hitting it a mech will spawn. Using the mech use the shield with the left trigger. Upon which he reflect roughly 10-11 bullets per run. Or at least that is what I average. From there just die and you'll respawn at the checkpoint and be right next to the mech suit. Repeat 4-6 more times and you should have earned another achievement in this craptastic game.