If you have not played the campaign and do not know what this is you should not read any further as it is a good surprise when you do find them.
There are various forms of flood and this achievement will come quickly because flood is always in big groups and are easy to kill. Using the shotgun has been known to be most effective, also the assault rifle to take out the groups of Parasites.
The first mission to encounter the flood is 343 Guilty Spark in Halo: CE, the best mission to kill flood will be The Library which is right after 343 Guilty Spark.
Below are some basic flood forms:
I found a great video telling information about the flood if you are interested, but do know this video contains MAJOR spoilers if you haven't finished the HALO 3 story. You will also get a good laugh out of this and do note that the video is not animated like the cover. Video was done by Machinima.