There is no way to actually tell what percentage your accuracy is at. You must beat the first 2 waves in arcade with higher than 25%. You can use a modification called "Battle Cry" after you buy it to help with this achievement. It slows everything down 50%. The trick is to let things hit your base and possibly even destroy one of your light guns in the first round. Try to only shoot when there are a lot of things on the screen at once. Once Wave 3 starts, don't shoot at all and let yourself die.