The various enemies of the game are presented in rough order of appearance.
Soldiers
Foot Soldier
200 points. Foot Soldiers are, by far, the most common soldier that you will encounter throughout the entire game. They move about the terrain, looking for a good position from which to shoot you. They also pack grenades just as you do, and not only that, but they can throw them at an angle. The only advantage that Super Joe has over these soldiers is his speed: he can maneuver around much faster, and he can fire his weapon more frequently. Foot Soldiers come in an endless supply. The enemy forces will never run out of them, so don't try to destroy them all. Just aim for the ones that get in your way.
Wall Soldier
500 points. Wall Soldiers position themselves behind low lying walls in an effort to shoot at you as you approach. Super Joe will not be able to hit these soldiers with his gun as long as they remain behind the safety of the wall. However, he can lob one well thrown grenade over the wall and take out the enemy threat. When Super Joe gets very close to the wall, these soldiers will abandon their post and retreat.
POW Guards
Early on in the first stage, and few more times throughout the game, you will come across a POW being led to captivity by two armed guards. They won't notice you coming up behind them which will give you an opportunity to shoot the guards and rescue the prisoner. The first guard that you shoot down is worth 1000 points, and the second guard is worth 4000 points. As soon as the two guards are defeated, the POW will run off to safety. And don't worry; you can't harm the prisoner in any way.
Mortar Soldier
500 points. Some soldiers can be seen manning mortar guns. They are typically situated behind walls, and can fire a series of mortar shells at you with deadly accuracy. As a general rule, you should never stand in one place throughout the entire game, but this is true even more so when a mortar soldier is around as you are sure to get hit. When you get too close to this soldier for the mortars to be effective, he will abandon his post and resume attacking you with a regular gun.
Rock Soldier
500 points. Some soldiers position themselves in such a way as to take advantage of cover provided to them by the terrain. As a result, it's not uncommon to find a soldier ducking down behind a rock. Like the wall soldiers, you cannot shoot at them from below due to the protection afforded to them by the rock. However, a well placed grenade can slip behind the rock and destroy the soldier. If you advance towards the rocks, the soldiers will eventually get up and retreat.
Commander
2000 points. The Commanders of the enemy army are peculiar soldiers. They wear a different color outfit and they are unarmed. In fact, once they appear on the screen, their only goal seems to be to escape unharmed. There is probably good reason for this, as their high point value will make them a worthy target for you. Hitting them will increase your score quite a bit, but letting them escape has no penalty, so don't put your life at risk just to shoot them.
Trench Soldier
200 points. As their name implies, these soldiers are found inside trenches that are dug out of the ground. They are afforded the same protection as rock and wall soldiers: they can't be hit with gun fire, only grenades. Approach them with the usual caution and beware of the bullets they fire when they poke their head out of the trench.
Bazooka Soldier
500 points. In the later stages, soldiers with shoulder mounted bazookas begin to make their presence known. Bazooka fire has the same general properties as grenades except they are much faster. As a result, you will need to react much quicker when these soldiers are headed your way. They have a very limited range of angles at which they can fire. They primarily fire straight down, although they can aim slightly left or right.
Water Soldier
200 points. Some soldiers of the enemy army will try to take advantage of the element of surprise. They will submerge themselves underwater, only to rise up and take shots at you when you approach. Your gun is ineffective against these aquatic enemies, but grenades can displace this threat if you can position yourself in such a way to hit them from below. However, they usually manage to hide their presence from you until you are roughly alongside them.
Tower Soldier
500 points. Near the enemy airports, you will find a series of towers with a soldier watching the action from above the ground. These soldiers will try to pick you off from their elevated positions. As usual, your gun will not penetrate their defenses, but a grenade can be tossed up and used to take them out. In general, these soldiers are typically positioned off to the side, and not worth the trouble to attack.
Gunner
200 points. At certain points throughout the game, a canvas truck may begin to drive across the screen, slowly stopping at certain points just long enough for a gunner to drop out of the back. These trucks can bring hosts of gunner reinforcements. Gunners are no different than regular soldiers, but their appearance right in the middle of the screen can disrupt your passage through the stage. Keep your eyes open when these trucks appear.
Vehicles
Bridge Motorcycle
At many of the bridges that you encounter halfway through each of the stages, it is not uncommon to encounter a motorcycle driver. He will drive halfway across the screen to stop in the middle and lob grenades at you as you are about to pass through the opening in the bridge below. You cannot attack this soldier, so simply avoid the grenades he throws and reach the safety of the bridge. These soldiers will not appear if the bridge is cracked.
Motorcycle
Later in the game, motorcycle drivers join the entourage of vehicles that may try to run you over as you cross the empty expanses found in certain stages. They cannot be attacked, and can only be avoided. Move quickly when they appear.
Jeep
1400 points. Jeeps will appear from the bottom of the screen with a driver in the front, and a gunner in the back. They move quite quickly and it's difficult to nail them with a grenade, but your machine gun can be an effective weapon against this vehicle. Just be careful to avoid the shots fired at you by the gunner.
Humvee
Like the motorcycles, Humvees will appear on one side of the screen and drive across to the other in an attempt to run you over. Be aware when they arrive and get out of their way as quickly as possible. Unlike the Jeeps, the Humvees are bullet-proof, and cannot be destroyed.
Turrets
Turret stations can be found along the side of the screen in portions of the later stages. They are heavily fortified, and the barrel pointed out presents a threat to your survival. They angle at which they can fire is limited, but once you're in range of a turret, you can't throw a grenade at it anymore. If you take the time to stand below the turret before you cross their line of fire, you can destroy the gun barrel with two grenades. You won't receive any points for the accomplishment.