Kill the upcoming enemies which will be targeting you to kill. You will have a gun with you and you can upgrade this weapon by earning points over the levels. Moreover, there will be few bonus weapons during every level. Grab them to develop your shooting skill. You will need to score up to a certain range to finish a level. You will have a health bar which will indicate how much health you have. Once this bar is empty, you are killed and have to restart the mission. There will be a number of enemies coming towards you and they will be pretty fast in killing. So you will also need to be accurate and speedy enough in shooting.