Metal Slug features a 'blood ON' option (soldiers squirt blood when shot, instead of water). This can be activated via the 'Setting The Soft Dip' option in the game's dip switch menu.
When you parachute into the level at the start of a mission, press B to make the parachute disappear, allowing your character to drop to the ground more quickly.
Don't waste ammo. Metal Slug requires some thinking and shooting indiscriminately is not the best way to play this game.
Don't waste special weapons on shields. Shielded soldiers will throw their shields aside when they need to fire, so shoot at them only at this instant. Or use your knife if you manage to get close enough.
Use your knife against lone enemies. There is no need to waste ten rounds of your Heavy Machine Gun on a single enemy soldier.
When Metal Slug is dangerously low on health (but not so low that you are forced to leave it), use the frontal attack to inflict major damage.
Whenever you jump out of the slug, you are invincible for a brief amount of time. This is especially useful against the blue mortars, since they can't be avoided. Note, however, that there is no period of invincibility when you jump back into the slug.
There is apparently no 'reload time' when you throw grenades or rockets. The rate depends on how many of either are currently on-screen so if you are close to an enemy, the rate will be negligible. You may use this trick against some bosses and in any other situation to pass through unscratched.
The best way to earn points in to rescue hostages, but this can be tricky. The first problem is finding them, the second is to stay alive to the end of the mission to get credit for your rescues.
As well as the easily-spotted hostages that litter the levels, there are also numerous hidden hostages:
MISSION 1 (hostages: 10):
Two pairs of are hostages hidden in the two waterfalls just before you get to the boss. Just blast the tops of the waterfalls and they will fall.
MISSION 2 (hostages: 12):
Blast down at the green bush underneath the bridge just before you reach the mid level boss (large tank on rails above you) and three secret prisoners will come running in your direction.
MISSION 3 (hostages: 15):
There are secret hostages hidden within the two large trees that you can blast down in the section where you have the Metal Slug tank. These seem to be random, just try blasting those trees down and sometimes they appear.
There are hidden hostages that drop down from the sky when you are fighting the end of level boss; just don't kill the boss until they have all appeared.
MISSION 4 (hostages: 20):
Just after you have got the Metal Slug tank at the beginning of the level, blast the ceiling of the bar just above the light/lampshade high up on the wall at the entrance of the bar.
At the end of level bosses you can either blast in the top right hand corner of the cave to get three hidden hostages OR blast the far bottom left hand corner for three hidden hostages. It is not possible to get both of these groups. You can only get one or the other.
MISSION 5 (hostages: 13):
Blast the first and second houses and two hostages will appear.
Fire at the top of the first and second trees in the stage in between the shops and houses. The first tree yields three POW's and the second only two.
Blast the undercarriage of the end of level boss after it has lifted up on stilts to get three extra prisoners who also provide useful power ups.
MISSION 6 (hostages: 25):
At the beginning, shoot above you to find one prisoner.
Destroy the big pack of rocks to deliver one POW.
Blast the small white cloud above you just after the boat you are on has run ashore to get two hidden hostages.
When you're out of the boat, go left in the water to find some points.
POWs appear on screen during the fight with the final boss, if you shoot in the top right corner.
Beat the game in two players mode to view a different ending sequence.