It's a prudent thing to keep your squad moving away from the enemy fire.
When you engaged an enemy, get out of range of their return fire as quickly possible, and be ready to move again once they've got their sights back on you.
If you don't want to spend real money in the in-app purchase store grinding a few of the vehicle-based missions can help out. It's very easy to rack up kill combos in these missions.
Keep your defensive and offensive capabilities leveled up evenly in tandem with one another.
Increasing your squad's movement speed and firing-range is a must.
Save your grenades and rocket launchers for taking out structures.
Always take advantage of landmines and explosive barrels when using these ranged weapons.