Actually a very easy achievement to get. Go into Player Match, Create Match, and select "CO-OP". Have your friend join your game and then score 20,000 points between the two of you. It's a combined score so no need to worry about getting 20,000 on your own. Some tips if you are struggling a bit:
1. In the IO tower, shoot as many spiders as possible before entering the tower as each spider is worth 50 points.
2. For the MCP cone, do your best to destroy all the blocks as that gives you a bonus of 1,000 points. If you miss one, then stay as far down and to the right as possible and keep shooting to the left because the blocks will cycle back around and you can sometimes get all the stragglers at the last second.
You should hit 20,000 somewhere in the second level, so if you can't make it to level two on your own then I suggest practicing until you and your partner can both get to that level. My friend and I made it to somewhere in level three and had 35,000 points between the two of us before we died.