A lot of people don't like Gun Mayhem, as I can tell from the comments and the rating, but i find that its a fairly cool game. It has a little bit crappy graphics but thats okay. Some other changes could be made to make it better too. But here is my guide:
First off, I recommend using the snake as your pistol and using the random gun perk when you get there. The snake is really powerful as long as you take good shots with it. Plus, its cool.
Level 1: This should be extremely easy so i wont help you there. Just aim and shoot.
Level 2: This shouldn't be too hard, but i recommend taking careful shots because he is on the moderate/faster side. This is where the snake comes in handy, and it still should be an easy level.
Level 3: Gun Games can be really fun, and they are my favorite. So what you do here is be really careful with your shots if you cant make it on your first try. This is not a hard level, but i can see how it could be frustrating. You need to be extremely careful because if you run out of bullets with your current gun, you get knocked back to the gun below it. So take time aiming and the enemy kind of sucks so you can dodge real easy. Just don't unload with the machine guns.
Level 4: You can be less accurate and careful with your shots in this level, because there is two enemies and so its easier to hit. But what you have to do is really be jumping around and dodging a lot of bullets. Because they both shoot quite a bit and the bullets come from all directions.
Level 5: I would recommend a stronger gun and not a machine gun in this level. Like a shotgun or a sniper if you can get one. He really isn't that hard of a boss. Just run fast the whole time and get out of the way of the bombs. he isn't too fast so you can unload a bit.
Level 6: this was a hard level at first, at least for me. what you need to get is a fast firing weapon, or a sniper. either will do. And so what you gotta do is either take nice and accurate shots with a sniper, or just unload on them with the machine gun. Jump around more than usual since there are two, and just let er rip.
Level 7: Not too hard of a level. Don't use a machine gun if you can, take a high power weapon like a shot gun or sniper. Sniper would probably do the best. Take good shots at him to not let all your bullets go.
Level 8: this is a tougher level. This may be a good spot to start using a buddy, either a computer or a person. Because it will get you extra lives and may kill the enemy. But ill tell you why they will get handy in a sec. What you have to do is fire either first, or at the same time as the pirate does, because you will kill each other. If you fire first at least one time, then you will win, because you will both keep dying. But, even if you dont fire first, you will win if you shoot at the same time the whole time, killing each other all the times. Because here is where the buddy comes in. at the end, the pirate will have no lives left, and neither will your first character. but you buddy will be there having lives left. That is the cheating way pretty much, but thats how i did it.
Level 9: Just watch his arrow where he lands every time, and you'll get him every time. I dont think youll need a buddy for this one, but you might want one if you cant do it alone. Jump a lot in this level, and then watch where he shoots from. Take a machine gun or shot gun to him and watch him fly. He isn't very smart, and cant come back to the platform very well once hes off.
Level 10: The big cahooka. Jump a LOT. in this. Pretty much any time of gun will work okay, but i would stay away from SMG's and shot guns. go with assault machine guns and snipers. If you have a sniper, jump even more and take accurate shots. If you have a machine gun, be more accurate than normal but UNLOAD, JUST UNLOAD. You'll kill him after a few tries
The controls are changeable. There is quite a few weapons.. most other shooting games have way less. I know i hate the shooting thing, it should just be point and click with the mouse. In the tutorial level, that just must be for you, because that didn't happen to me, and i didn't see anyone else say that.