Tips and Tricks
Remember that for most weapons you'll need to judge where the shot will land, so don't aim directly at your target.
Always be researching new weapons and munitions so if you can't progress start grinding for currency to buy better weapons.
The key is to continue moving around the map. Enemies strike from different directions, and if you don't keep up the tactical advantage, you're likely to get overrun. The goal is to keep your base intact throughout the course of the mission.
Chain guns are quite effective if enemies get close, as they can wipe out small soldiers and vehicles with ease.
Small missile launchers are quite powerful, but the big boys are the ones you'll want to use, especially if you're trying to wipe out multiple targets at once.
Medals act like Achievements, and unlock as you meet certain tasks in the game.