In order to get this achievement, you will first need to beat the 50 waves once (beat the main "story"). Once you beat the original 50 waves, you will be given the option to replay any wave again by selecting it. Once you select the wave, you will need to beat the wave, and then repeat that process for a total of 99 times. For this achievement, select the first wave as it is the easiest/quickest (although it doesn't matter which one you pick if you want the full 200/200 as you will need to do this for all of them). Simply beat the first wave, then go back to the selection and select it again, beat it again, rinse, repeat. This is a major grind, but with an average time of about 1 minute for wave completion/start up again, you are looking at about an hour and a half just for this one achievement/level.