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Even though you have an unlimited amount of lives, all of the things done before that will not count after you die; try not to die while fighting the Boss.
In mission 5, jump into the plane slug. Now as soon as you start the level proper, press L to crash the slug. You will eject and go onto a different route where you parachute down into the enemy docks.
Every time you rescue ten prisoners, you will get a card.
You will get soldier cards when you kill 100, 200, 300, and 999 soldiers (for a total of four cards).
There is a card in the beginning of mission 4 called Type R. If you equip the card, your Metal Slug turns yellow and the gun moves in reverse. For example, if you are moving forward your gun will point in the opposite direction.
Collect all 99 cards in the game to get the final 100th "All cards" card.
You need the "paper thin" card. Turn it on, then complete any mission. With paper thin turned on you will die with just one hit. The easiest way to get the decoration card is to complete mission 2, mainly because there is a Metal Slug at the near start of the level which is shielded, so if you can manage to get through the level with the Metal Slug it shouldn't be as difficult to get the card.
You just have to clear the mission, you can die as many times as it takes: