This is like the Sharpshooter achievement except you'll need a higher percent and get to wave 5. You can stack this with Special Forces, which is the other accuracy achievement. I suggest going for Special Forces and you should be able to pick this up along the way. You will want to use the Battle Cry modification to slow everything down. This can be done with max upgrades. Just be selective on how you shoot. You will have the chance to shoot a box that moves slowly on the screen. The more you shoot it, the more your percentage will go up. You can also do this with a boss. Make sure you focus all guns on them. If they move, make sure to move with them so you don't start missing.