Kill the strongest unit out there before turning to fight the others. Abiotic enemies - Always concentrate all firepower on them if neccessary. Helicopters won't stand around attacking you in the first parts of the game. But they'll get to screw with you later in stage 3. As for tanks, shoot them down and take out any interfering soldiers.
Same thing goes for turrets etc.
These are the worst shooters in the game. But are good for an extra 5000 pts. AND they can run and escape like mice. Try shooting them in the head and upper body to grab additional points. Before inserting the last token, hold the trigger and step on the pedal.