Just to add to the 3 solutions here already. Finally after 3 years having this game this and Lair Slayer was my last two cheevos to complete the game. The method that I used to tackle the cheev was probably the most easiest in my honest opinion!
- First pick co-op, easy, area 1.
- Both players play normally til you get to the boss.
- When you reach the wall boss just die til you restart at the beginning of area 4.
- Now make sure the player who needs the cheev, doesn't fire a single bullet. For some reason when you respawn your accuracy percentage resets.
- Have the other player helping out shoot everything at the beginning. Move slowly and not too far ahead. Stop at the area where there is a building on the right side with the grenade launching enemy. Destroy those and the motorcycle guys to your left to make sure the area is cleared up. Then move up slowly and there will be a aircraft to your right side dropping normal infantry down. DO NOT KILL THE AIRCRAFT AND INFANTRY!
- Have the player who needs to get the cheev get out from the vehicle and line up to the left side of all the infantry dropping down and shoot to the right with a single bullet. Since there are multiple infantry you might be able to kill 2 to 3 with one bullet. If you miss then you have to die and respawn at the beginning of area 4 again. It's pretty hard to miss since there are 5 to 6 troopers dropping down and they make pretty much make a wall for you to shoot at.
- Now make sure the one who needs the cheev deosn't fire anymore bullets and to just dodge every bullet and enemy out there. (You can run over infantry, as it doesn't count towards the cheev) Advance and have the other player kill the wall boss. At the end of the level you will get your cheev at the review screen. You should be able to get a 100% since you only shot one bullet.
- If other player needs cheev then rinse and repeat!
I have tried this on Xbox Live co-op but I cannot confirm this with local co-op. Please comment or msg me if you find this possible and I will edit.