Pick ranked deathmatch and start up a 3-4p game. Have team join in through ranked quick match and get as many kills as possible. Dont need default settings and you can change the time or limit to make it so you can get max kills for your time. You get the acheivement on the final 1000 kill in game not after the match.
NOTE: NEVER pick the GREEN ship... this will corrupt the save of the match your playing as its poison will get you kills after you die and if you die from your own poison the game cant register it. This VOIDS all kills in the match and you need to start over again from scratch. Check leader boards after each match right d stick to see death match if it tallied up.
It takes 4-6 hours depending on how you all play together. Light blue ship is the most powerfull and best normal shot.