After the mad doctor and his goons kidnap Mary, it's time for our heroes to bust into the airport and kick a little ass! The first action you should set about is destroying the van that's blocking your way and deploying goons into the vicinity. After that, the door to the hangar is blocking your way, though an eagle flies by with a chest containing a Bomb, so that should assist in taking care of it and the enemies.
Inside, you can get yourself a Fire in the first chest, and afterwards you'll encounter a large goon, who requires more hits to take down than the smaller kind. A wall of barrels with knife-flinging baddies is the next obstacle; the knife guys are pretty devious if you're unprepared, as much like the soldiers from Metal Slug Advance, they'll just keep slinging knifes like no tomorrow, though you can aim diagonally in the game. The Bombs are best for disposing of the barrels.
Another wall of barrels blocks the way afterwards, with only one knife guy, and afterwards it is primarily pipes and a few more muscular and knife guys. Hanging around this area really only allows more guys to pop up from the entrances, but rushing is likely to get you clobbered by the big dudes, so tread carefully yet with haste. At the end of the hangar, a huge skull bomb drops from the roof...
And blows you onto a flying plane. After a few generic opponents, you'll come across the doctor, who launches bombs into the air to shower down on you, and jumping to the other side of the screen when hit, though just two wallops is all it takes to send him packing.
He then boards the separated cockpit of the plane, which flies back and forth dropping a regular goon and a knife goon down each time. This is the perfect opportunity to use a super attack, which takes off half of the health of the boss and sometimes more if it's a good one. Even without, this boss is simple and should go down in no time.