As you make your way through levels, you'll notice that jumping and shooting simultaneously, shooting diagonally, and shooting downward are all important skills to master. Since the game features onslaughts of enemies coming almost nonstop, this ability to shoot in all directions is an absolute necessity. Also, I suggest flipping on the auto-fire option, which lets you hold down the Fire button to shoot continuously.
As is the case with other shoot-'em-ups, finding new weapons in Metal Slug is one of the most fun parts of the game. First, you start with a relatively weak pistol that can only take out enemies one by one, with one bullet at a time. As you make your way through levels, though, you'll encounter prisoners who will give you upgrades when you free them. My favorite weapon upgrade is the shotgun, which has a short range but can take out large groups of enemies all at once. There are also powerful machine guns, flamethrowers, and other weapons. In addition to guns, you'll come across vehicles that you can use to get through tough spots. For instance, in the first level, you can jump on a camel outfitted with lasers, which happens to be a perfect "vehicle" for attacking the giant airship on the following screen. There are also tanks, robotic suits, and vehicles.
One of the coolest things about Metal Slug 2 is the Multiplayer option. Using Bluetooth, you can connect with a friend to play cooperatively and simultaneously. Just like in the arcade version, you'll have to back each other up and share upgrades in order to succeed.
Its worth noting that Metal Slug 2 can be extremely taxing on your device's RAM, especially during more advanced levels with lots of onscreen action. On one of my devices, I experienced several forced shutdowns due to the overload. That said, another of my devices was able to handle the RAM demand just fine, so the game's performance will depend on your device's specs.