This walkthrough is written by Rage Quitter 87.
Mission 2 is possibly one of the funnest in the game, mainly because you can use the Metal Slug so much. There are also lots of useful cards to pick up in this mission. In terms of difficulty this mission is easy if you use the Slug in it.
Part 1 - Mountain path
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 When you start, move right to see
a Slug and two enemy soldiers who are talking to each other. Kill them both then
jump into the Slug. You can use this Slug, as long as you don't let it get
destroyed, for the entire mission unless you go into the secret area, but more
on that later.
 Keep moving right and shoot down any enemy soldiers that get in your
way. You will arrive at a large log which has half of it sticking out into the
air. Shoot the tip of it and prisoner (016) who will give you Card 013
I.L. Clip. Once you have saved him and got the card, go back up and drive up
the log, then jump off it to the right whilst shooting in the right/up right
direction to free another prisoner who was tied up in the air. If you can't save
him, get Card 039 Thruster+ and do this mission again later, you will have a
much easier time getting this prisoner. Anyway, this prisoner, prisoner (017),
will give you Card 015 D.S.Clip.
 Go right a bit and the screen will lock as one of those tall enemy
tanks rolls into view. Blow it up and the screen will unlock, so move to the
right and you'll run into prisoner (018) who has a Heavy Machine Gun
for you. Keep going right, slowly. You will see the tip of a turret gun. Stop
moving right, and jump and fire a shell to hit the turret. The turret will fire
a shot at you, but you can't be hit as your are too low. Once the shot has gone
past you, jump up and fire another shell. Keep repeating this process until the
turret is destroyed.
 Now move to the right and the screen will lock once again. Enemy
soldiers will throw bombs at you and a enemy helicopter will appear and start
dropping bombs in sets of three, as usual. If you want to have full shields for
the slug, jump out of the slug and use the heavy machine gun to destroy the
helicopter and kill the soldiers, then jump back into the slug. If you do use
the Slug and it gets damaged, destroy everything on the screen, then shoot at
the black square in the destroyed turret and prisoner (019) will appear,
who has some gas for the Slug.
 Jump upward and you will see another log, this time pointing out to
the left. Jump up to it and go the left of it, where there is a box with some
bread in it, then jump and shoot directly upward to free prisoner (020),
who has Card 070 Marco.
 Now start going right again, and go up the next log you see which is
longer than the previous two. At the near tip of this log is prisoner (022),
who will give you Card 032 L. 1 Armor. From the tip of this log, jump to
the right and jump up a couple of ledges. To the left of you there is another
log which is the longest yet. Drive up it, and at the tip of it is prisoner
(023) who has a Rocket Launcher for you. Now drive to the tip of the
log and jump off it too the left whilst shooting in the up left direction to
free prisoner (021), who was tied up in the air. Save him to get Card
 Once you have saved this prisoner, keep going right and jumping up
ledges, until you get to a level patch of land. The screen will lock and a enemy
tank and small helicopter will appear, destroy them both to proceed to the next
part of the level.
Part 2 - Treetop village
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 When you first enter this section, there are two large trees in the
background shoot at the middle of the second one and Card 074 Rumi will
drop down for you to collect. Go right until you see a platform in front of you
which you have to jump across too. Before you do, a diver will jump out with a
big missile, kill him before he has a chance to throw it at you. When you do
jump onto the middle platform, another diver will jump out, so be ready to shoot
him. On the platform there is prisoner (024) who has some fire bombs
which replace your grenades with bombs that spread fire everywhere. When he has
been saved, jump and shoot directly upward to free prisoner (025) who has
a Heavy Machine Gun to give to you.
 Once he has been saved jump across from this platform to the right.
Continue right killing enemy soldiers and you will eventually arrive at a ledge.
Shoot the tip of this ledge and Card 078 Metal Slug will drop down. At
this point, you can drop down the first pit you see to find an entrance; if you
shoot the root of the lowest platform Card 020 R.Launcher+ will drop out. The entrance leads to part 3, but if you want to hold onto your Slug, just ignore the pit and keep heading forward.
 You must jump over to a wooden structure, and the screen will lock as
a enemy submarine drops down and starts firing missiles at you. Use your Slug's
shells to destroy it quickly. Then jump across to where it was (the screen will
be unlocked) and jump across again to yet another wooden structure.
 You will now see a new type of enemy who have swords, can throw bombs
at you and, worst of all, will blow themselves up when killed which will hurt
you if you stand near them.
 Anyway, on this wooden structure, shoot the enemy high above you,
then shoot at the left tip of this wooden structure to reveal prisoner (026),
who will give you Card 058 Amber Jewel. After doing this, free
prisoner (027) who is directly in front of you, who will give you a
 Jump across to another wooden structure and a few enemy troops and a
small helicopter will attack you, take out the helicopter first. After they are
gone, jump across to the next wooden structure, which has a few more enemy
troops to kill, then jump across to yet another structure. Another small
helicopter will appear, destroy it and then jump across to the last wooden
structure which has prisoner (028) on it, save him to get a turkey.
Continue right to finish this part of the mission.
Part 3a - Dynamite cave
Inside there are two prisoners above you on platforms, as well
as a couple of knife-wielding soldiers. Free them by throwing fire bombs on each
platform. The one on the right, prisoner (029), will give you the
Laser weapon, whilst the other one, prisoner (30), has some canned
food. Now go to the far left of this room and aim upwards; starting shooting
and Card 003 S.S.Sword will drop down.
Part 3b - Boss
The boss of Mission 2 is called Emain Macha, also known as "Twins" and is a group of two tanks that attack you one at a time.
The first one is slower and will trundle on screen. The only place it can be
shot at is it's turret, so constantly fire at that to damage it. The turret will
fire fire bombs around the area so tread carefully to avoid them. Also,
sometimes an enemy soldier will pop out of part of the tank and shoot at you,
kill him so he doesn't cause trouble. Eventually the tank will explode.
However, once it has exploded, the second tank comes on and it moves a lot
faster than the first one but has the same tactics and weak spot as before.
You'll just have to have quicker reactions this time. Once this tank has been
destroyed the mission will end.
On the Hard setting, the second tanks moves around ridiculously fast and thus
is harder to land shots on. If you want to defeat this thing quickly to get it's
card, make sure you have your Slug with you at this boss (the Black Hound is
useful against the first tank, but the second one can be difficult to land hits
on with the homing missiles, especially on Hard mode) and use it's turret/homing
rockets for a quick and easy win. If you don't have the Slug, then my advice is
to use the Laser to destroy it, you can get another Laser Shot by killing the yellow
troops that appear from either form of the boss.
Click maps to enlarge them.
Identical in layout, except the prisoner carrying gas is tied to a pole in the prototype, rather than in a hidden hotspot.
It's possible for the second stall point to continue without the cannon being destroyed. Destroying it will only remove the cannon, not change the background graphics; the 'destroyed' graphics are embedded off-camera in the background tiles.
The cannons notably feature spiked tops, suggesting they might have damaged you before they were destroyed, but there is no hitbox in place.
Largely identical layout-wise.
The stall point with the suspended submarine happens slightly earlier, over the drop leading to the cave, though it's very easy to force the camera elsewhere and remove the boundaries. It's one of the more functional enemies, with all attacks appearing to function properly and the player able to bounce off it (and potentially skip it entirely). Damaging and destroying it will display no explosions however, it just erodes before eventually disappearing.
The Japanese soldiers will run at the player with their swords raised in an attempt to melee attack, but it and their bombs don't appear to have a working hitbox yet. They're prone to running off ledges while chasing you, or turning in place perpetually until you make a move.
In the final, the Japanese soldiers will only run at you in the dungeon; in this area they prefer to attack from afar.
The wooden scaffolding has extra background details: storage nets or planks with knifes and weaponry stowed in them. These are solid objects with collision boxes that will block your shots, but not anything else, making them particularly bothersome when fighting soldiers or helicopters. These were removed from the final version.
The cave has no enemies and an extra card. The perspective on the platforms was altered significantly, but otherwise the graphics appear to be identical in here.
The boss is unfinished, and will simply sit in place while running through its animations on loop. It cannot be interacted with, and loading this sprite will permanently print those coordinates onto the screen; they can only be removed by getting a Game Over or resetting.
The background doesn't have the flooring baked in, but instead a small chunk of log bridge is floating in the middle. It's on the sprite layer and overlaps the player's sprite; it's possible the flooring might have been destructible, but that's purely conjecture.
Walking off-screen to the right will take you to Mission 3.