There are a lot of weapons that are given to you in the game, but there are also five secret weapons that you can find to boost your weaponry. The first is the throwing disc. You can search this after finishing the forest level and go to the second route towards the industrial area and find your way to it. The second item is the hand cannon, which you will get after finishing the exterior hospital area. Then comes the shuriken, which can be traced on the route towards Suburban when you complete the cemetery level. After that, spear is the last weapon and can be found on the underground route at the start of the game near the crash area.
Kill all the aliens
If you want to beat the game in the fastest amount of time, or only wish to check how soon you can blast through the alien boards, remember that the aim of the game is to finish off the aliens that come your way. However, if you feel the human military units is too much for you, then just try to run past them or sneak your way through them. You are not required to destroy these aliens to finish the game. Moreover, if you are stuck with army soldiers that will waste a lot of time, just go through them. Check your health status and use the fountain of health if you suffer damage.
Fountain of health is a very good trick and will help you if you are close to ending game. If you are in a fight and are suffering too much damage, then go to the map and select the option to restore your health. Your health will be replenished at the cost of your energy ousted for around 10 seconds.