Stuck in the game Mr. Schyzo? no problem here you will find a solution to all your problems ;)
To beat the levels that ask you to perform head Shots theres 2 easy ways to do it.
For the first one you will need the Revolver fully upgraded (No the Dual revolver) its the weapon with the highest chance to perform a Head Shot. Also is the weapon that gives you more money per kill, so if you are needing some extra cash to buy the bazooka, start killing with the revolver in the first levels will be a good idea. So just stand on a cliff wait for people to come and thats it. If theres no cliffs just arm your revolver, jump and shoot.
The second way is, you start killing as much as you can and when you get enough rage you go fully in berserk mode, stand on a cliff and shoot or start jumping and shooting. While on berserk mode you have really high chances to make a head Shoot or a Critical.
To beat the choppers theres this strategy, go to the edge of the scream, arm your most powerful weapon and start shooting and jumping, you will lose some HP doing this but this is the safest and best way to complete these levels.
You can make head shoots to any one in the game even the choppers. You just have to aim to the right place and you will have that shoot.
Theres really not a best or a worst weapon, all weapons have its advantages and disadvantages. For example the Revolver is the best for head shoots, but has a really long reloading time and the Machine guns has great reloading time but sucks at head shoots. My personal Favorite is the Sawed shotgun and the Tactical shotgun. Try all the weapons and decide for yourself. Post here and let me know yours and why :D
If you want to make money fast in the game theres some upgrades you will need to max. The Gold Amulet and the Shirt Strips (the orange strips) also check on your favorite weapon and see if theres an upgrade that allow you to increase the reward you get for killing with it. The Revolver, The Dual Sawed Shotgun, The Submachine gun, The Assault Rifle and the Rocket Launcher all have this upgrade.
Keep making Criticals and Head shoots and always try to get your combos to the Max.
The weapons with the higher chances to land a critical are the shotguns all of then including the sawed ones and the Rocket launcher that can be upgraded to have a critical rate close to the 50% this one is the best weapon to take down the choppers and the Shotguns and the Assault riffle to bust vehicles.