Metal Slug 3

Hint: Stage 1: Hidden areas:
Near the beginning of the first stage, you will see a submarine floating in a pool of water. Hold Down to walk into this area and hop into the sub to go to an underwater part of the stage. The next hidden area is at the part where the boat is located. Instead of getting on it, hop onto the ledge above it to find a secret hatch. Go inside to enter a cave-like part of the stage. Note: Every time you complete a secret area, you will go directly to the Crab Boss --- you cannot go in both hidden areas during the same turn.
Information in this section was contributed by ShoemakerR.

Hint: Stage 2: Get Thunder Cloud:
In the stage with the zombies, you can get the very helpful Thunder Cloud. Around the middle of the level, you will see a spot on a low area of ground where two prisoners are next to a man in a business suit. Hold Down/Forward to crawl down to this area and save them. Note: Enemies cannot get you down here. When you rescue the man in the suit, he will pull out a cell phone and call over two bodyguards with suitcases which contain a gold coin and the Thunder Cloud. To use it, just shoot your gun when the cloud hovers over an enemy.
Information in this section was contributed by ShoemakerR.

Hint: Stage 2: Hidden area:
The level containing the zombies has a secret and unique area. About the end of the middle part of the level, you will see a sign above a frozen-shut cave reading "ICEMAN". Walk to the left to go over here and shoot the ice wall blocking the entrance. Inside are numerous "ice men" and at the end of the cave is a buried Elephant Slug, frozen in the ground which you can free and ride.
Information in this section was contributed by ShoemakerR.

Hint: Cure zombie or mummy status:
To turn back from a zombie to a human, use the Med-Kit item found in enemies or boxes. To turn back from a mummy to a human, find the bottle of Holy Water found in enemies or boxes.
Information in this section was contributed by ShoemakerR.

Hint: Helper monkey:
Throughout the game you will find a wooden crate containing a monkey wielding a gun who will help you fight enemies. Note: Do not rely on the monkey helping all the time. Sometimes he will stop to pick his nose and do other things.
Information in this section was contributed by ShoemakerR.