There is a large tunnel at this part of the game, and you need to bring in a certain ally to battle. To choose your allies, hit or on the mech config screen to get to the ally page. Pick Luven Alhabi - the guy with the sunglasses - then play the level. He'll lay mines down along the tunnel, and you need to keep him alive. He's pretty weak, especially if you haven't leveled him. So bring some REP kits along.
To get past the boss with the superlaser, you once again have to call upon an ally to help you. This time you go find this him instead of selecting him from the ally page. Go to the Middle East, and go to one of the levels in the upper right corner. Play through it, and you'll find a shield-type Panzer. Bring him into the fight to hide behind when the boss uses the laser. His shield has a finite amount of energy however, so be sure to toggle it on and off in the command menu.
Beating the game
Once you're done with all the levels, you need to pick up a new carrier. Go to Western South America to get it, and then go to the flying fortress that zooms around the map.
The best place to shop is in North America, as they almost always have the most up-to-date stuff. When you want to save a little bit of money, the Middle East shop offers a 10% discount on everything.