Beat the levels corresponding to the difficulty.
Easy - Levels 1-6
Medium - Levels 7-12
Hard - Levels 13-18
The levels are considered complete when you reach the stop sign and have shot at least 80% of the targets in that level. If you are given the option to continue on the stat sreen the level is complete. Pretty simple game, the last couple levels can be a bit tricky and take a few tries to learn where everything is.
Please note that all achievements unlock once you fully exit the game.