Curved Space

I got this on casual and the level disc. I got this on accident and not sure why i did get it, it may be glitched. I played survival and did not shoot at all the whole game, was going for a different achievement that never did pop. After 5 little rounds or levels this rampage bonus achievement popped. Also, every 5 levels the difficulty ups its self. I hope this helps and works for others. I just dodged everyone i could and didnt even do a good job of that.