When you find the handgun, you automatically think that it will make fighting those monsters easier....
Well, it does make it easier, and quicker.
But if you do that, you wont survive the game. Why?
Well, because I made that same mistake, too, that's why.
My mistake was that I should have not used the gun on any monster you met, and just save up on bulletclips, and use it at the best moment.
Which is the final boss fight.
Those other monsters you will encounter can easily be defeated by just beating them up with your fists.
But the final boss battle, that's when the gun is the best item to use against.
Infact, it's the only way to fight him with, and the only way to finish the game, even.
Because beating him up takes too long, and you probably get hit many times by him, aswell, especially when you only have 500 seconds to go until the building blows up, and you have to fight him 2 times, which doesn't leave you much time to get out of the building alive before it blows up.
You'll never make it by just beating him up with your first, the handgun is the most important item to use in this battle, and it's the only way to complete the game on time.
Make sure to shoot with accuracy, don't miss or shoot around, because you will be in a serious predicament if you ran out of bullets during that battle, use the right moment to shoot, and keep holding X to fire, and release X if he suddenly moves out of the way, or anything in which case you don't seem to hit him.
Do it right, and you will have just enough bullets to defeat him twice like that, although, it could happen that in the second battle with him, you may ran out of bullet, and have no other option then to fight him on with your fists, but I'm sure if you do it right, then you might be able to defeat him with a couple of punches, and still have enough time left to lure him to the back of the plane and burn him alive with the plane's after burner on the third battle with him.
So, whatever you do, never used the handgun until you reach the final battle, and try to find and save up on BulletClips as much as you can.
In a way, the biggest puzzle in this action/puzzle game, is the handgun, because if you figure out that you can better save up and use this gun only at the best moment and time, then you will finish the game.
If you didn't, then you can start all over, like me. XD