Robotron: 2084

In Co-op, one player moves the character while the other shoots. You change roles when changing levels. You will unlock this achievement after completing level 5 (at the start of level 6).

Make sure you save people to make points, which grants you extra lives. Quickly kill red orbs, as they will transform into robots that shoot lots of killing stars. Once they're taken care of, save people and mop up the rest.

The solution by Mtld for the achievement works well. I will add that, if you're lucky enough to a local gamer who can help, it is faster to communicate with your partner if they're in the same room with another XBOX than trying to do it over chat because of the latency in chat. The CO-OP must be done over XBL though.

You can also do get the achievement by yourself if you have two XBOXes and two XBL Gold accounts and two controllers (you only need one purchased copy of the game). This is how I did it:

Login as your main XBL profile on your main console (the one that owns the game):

There are likely other ways of achieving steps 1 to 3 so choose your preferred method, if you don't like mine. This assumes your main XBL profile is on the hard drive of the main console.

1. Transfer your game to a USB stick using System Settings -> Memory -> Games -> Robotron. Then remove the USB stick after it has finished moving the game (wait a few seconds).

2. On your PC, go to XBOX.com and then to Your Account -> Purchase History. Find your Robotron purchase and choose to add it to your queue. On your XBOX, Choose to download it to your hard drive and wait for it to complete.

3. Re-insert the USB stick and move your main XBL profile to the USB stick. Make sure the second XBL Gold profile is on the hard drive of the main console (transfer it from the other console if necessary using the USB stick). Remove the USB stick from the main console and put it into the second console.

4. On the main console, login as your second XBL profile and start the game. Choose XBOX Live Game -> Player Match -> Private Match -> CO-OP (also press 'X' to make sure you have the defaults (3/25k/1) because you can't get the achievement otherwise).

5. On the second console, login as your main XBL profile and start the game.

6. On the main Console, send an invite to the main XBL account, which is now on the second console. After the main profile joins the game press 'A' a few times on both consoles to start the game.

7. Finish Level 5 (see Mltd's tips on enemies). The achievement pops on both accounts.

Tip: Swap the controllers between hands after each wave so that the one on the left always uses the Left Stick (Move) and the one on the right always uses the Right Stick (Fire). I found this easier than trying to using the opposite sticks to normal on the even numbered waves.

Good luck!

If you have any questions, please message me on TA.

You can also use this for the CO-OP Wave 10 achievement if you're good enough. I'm not yet. [ I've also posted this solution there in case it helps someone ]