Easy :)
Many people have wondered how to defeat Satan. Here is the answer!
You only wait until he walks on the ground then you use earthquake (Key 3) and shoot him! Many people ask me: "Isn't it better to shoot the people." The answer is: "NO!!"
Then they ask: "Why not?" Answer: "Because you won't be gaining anything because you have to defeat Satan."
It is a nice game. Not so good graphics though. Could be better.