The majority of the enemies that you will face throughout the game are the Wingates' low-level hired gunmen. They can appear on the screen from any number of directions (although primarily from the front in the beginning), and they tend to stick around until either you or one of them die. They fall down after only one shot. They're not very fast, but they are trigger happy and will fill the screen with bullets if you let too many of them survive. The gray ones approach you from the ground while the purple ones tend to shoot at you from balconies and cliffs.
Riflemen are a bit more sophisticated and discerning than Gunmen. They make their appearance on to the screen, shoot at Billy three or four times, and then make a speedy exit. They're not there to fight with you for the long haul. They sometimes appear in groups, and they take more than one shot to kill. The purple riflemen appear from the top of the screen, and retreat back to the top after firing. Alternatively, the gray riflemen appear from the sides, and retreat to the side they appeared from.
The Wingates have many snipers in their employ. For the most part, they set up in second story windows and shoot down at you. While they have a fixed position, and cannot move, they are no less of a threat as it's very easy to miss them or fail to notice one of the bullets that they have shot at you. It is definitely in Billy's interest to remove them from the scene as quickly as possible. They go down in one shot. The building snipers (shown right) appear in many stages, but the native snipers that peek out from inside their tents (shown left) appear on stage six.
Bombers look like large punks who where green and toss sticks of dynamite on the ground around you. They approach from the top of the screen to some point down below, stop and prepare a stick of dynamite, and then toss it in front of them. The dynamite lands on the ground, and flashes briefly just before it explodes. The explosion will easily kill Billy, but fortunately for you, it's capable of killing other enemies as well. If you lure an enemy onto a stick of dynamite when it explodes, that will be one less enemy for you to worry about. While the dynamite is on the ground before it explodes, Billy can make it disappear by walking over it, presumably picking it up although there's no animation for this and he doesn't actually throw it back. Bombers fall down with one shot.
Back-stabbers are one of the few enemies that do not shoot at you. Instead, they appear from above, on balconies or cliffs, and jump down to the ground. From there, they proceed to jump across the screen with two hops in whatever direction they are facing. Their goal is to pounce on you and stab you with their knives. Don't give them the opportunity, just get out of their way. It is possible to kill them, but it is difficult as it takes many shots to put them down, and they can only be struck will standing on the ground.
In the sixth stage, when you pass through a native American tribe's territory, many natives will appear to defend their homeland. With the introduction of firearms, a majority of natives will choose the effective and deadly pistol as their weapon of choice against you. They behave predominantly like regular gunmen.
While many natives prefer to use a pistol to stop you in your tracks, quite a few are still skilled and proficient with the use of bow and arrows. This more traditional weapon is just as deadly as bullets and they can are often fired at Billy quite rapidly by surviving archers. The one weakness that they have over the pistols is that the arrows can actually be shot down by Billy's gun if he is lucky enough to strike one with a bullet.
Native hatchet thrower
There is a third variety of weapon that the native Americans will use to defend their territory: the hatchet. Practiced warriors can throw these small ax-like blades with tremendous force. The hatchets are large and they are thrown fast. Though they can be shot down like the arrows, you shouldn't count on your ability to react quickly enough to them given how much activity will be going on around you when the hatchets appear. Instead, take out the source as quickly as possible so that you don't have to worry about dealing with the hatchets at all.
Native elders behave in a similar manner to the riflemen. They appear from the top of the screen, quite often in groups, and make their way down to a certain point before stopping. Then they fire off three or four shots at Billy. However, when they resume moving, they actually tend to continue moving down, instead of retreating to where they came from. Take care when you see these enemies, and avoid their attacks.
Less of an enemy, and more of an innocent bystander, bulls are not a threat unless they are threatened. They are content to peacefully stand around unless you happen to shoot them. If you do, they will start moving down the screen. In fact, the more they get shot, the faster they will move. Regardless of speed, if one touches you, you will get trampled or gored by them, so steer clear and let them run past you.