here's the guideline to save all prisoners:
105 total prisoners
Mission 1 (10 Total Prisoners, 4 Hidden Prisoners)
Just a little, before the boss, there are two hiddens prisoners for waterfalls (4 in total)
Shot up repetedly to find em out.
That's all for the first stage.
Mission 2 (12 Total Prisoners, 3 Hidden Prisoners)
After the bridges section and just after destroyed the tank, you have to shot down to the little bush to find three prisoners with a medal each one.
Mission 3 (17 Total Prisoners, 4 Hidden Prisoners)
At the boss we have 5 prisoners that are not really hidden.
Basically you must wait without kill the boss and they fall down from trees.
Before the boss in the forest section, there are two trees to destroy. CPU randomize the discover of 1 or 2 prisoners for each tree.
Mission 4 (23 Total Prisoners, 11 Hidden Prisoners)
Start of the stage, don't move and shoot in front of you to the hovel killing all the soldiers that appear continusly. Around 54 second three prisoners fall from the sky with machinegun, bombs and a chicken. Pay attention to the aerial raid.
When you got the Metal Slug and meet the first stair, shoot up placing yourself in the middle of the stairs to reveal 3 prisoners.
At the bunker there's a visible prisoner to save, but if you wait for 4 soldiers to come and you kill em, this reveal 2 more prisoners.
Then, fighting the boss there are 3 prisoners in up-right side of the screen and other 3 in low left side (they''ll come from the gallery) but you have to kill the tank below without killing the tank above.So go revealing the prisoners up right and kill the tank below, then go down to reveal the prisoners in the gallery e finally kill the tank above.
Mission 5 (15 Total Prisoners, 5 Hidden Prisoners)
Destroy the first two houses hitting the roof , reveiling two hidden prisoners.
A few step after, shoot up the second big tree near the zone where you find the metal slug to reveal 2 prisoners.
Fighting the boss, destroy his lower part (the one that throw flames) to reveal 3 hidden prisoners.
MIssion 6 (28 Total Prisoners, 6 hidden Prisoners)
Just reached the bridge stop on the first floorboard shooting on the left to save 1 prisoners then go on till the tenth floorboard and again, shoots to the left to save other 2 prisoners.
Shoting on the rock (or using a granade) you can save 1 or 2 prisoners (random decided by CPU) but there's a lot of events choosed random by CPU including no prisoners and 1 hidden soldier or no prisoners and 3 soldiers throwed across the screen.
After completed the submarine section you can shoot to the white cloud to reveal 2 prisoners.
Fighting the boss place yourself on the right corner of the screen and shoot up. You have to wait till 6 prisoners come down one by one.
sorry for the translation errors
all credits go to Gemant