Learn to be accurate with all of your weapons. As the enemy raids get larger, longer, and more difficult, you will need to conserve ammunition. With many of the explosive ammunition types, it is possible to kill many enemy paratroopers with a single shot. Conserving ammunition will also reduce your supply expenditures, making it easier to buy the really effective (and cool) weapons. Finally, if you are accurate with your weapons, you will be able to fire at one target and immediately move on to the next without waiting to confirm the kill, saving time (a precious resource during the later raids).
Kill enemy commanders as soon as they appear. As you kill commanders, you recieve a scoring multiplier (displayed in green next to the score display) for the rest of the current raid. Also, if a commander lands, he will begin inspiring the other enemy troops to greater and greater efforts (mostly by shouting "Move it!" a lot). The enemy forces will move faster and fight more effectively when commanders are on the ground.
Kill special forces troopers quickly. These troopers are much more effective at fighting your ground defenses then are the regular grunts. Also, if a trooper reaches your gun emplacement, he will inflict LOTS of damage. 'Nuff said.
Watch out for the combat engineers, especially in the later raids. Their demolition charges can quickly reduce your defensive emplacements to rubble. If you have an area-effect weapon or the rapid-fire Dual Gun, it is possible to shoot the falling demolition charges before they hit the ground. Try it sometime. Also, listen for your troops yelling warnings as the troopers appear and prepare to drop their charges. Consider saving up to buy camouflage netting for your gun or your ground defenses to reduce the liklihood the engineers will be able to spot your defenses and hit them with their charges.
Equipment Purchases:
Don't buy something just because you can afford it. Assess whether a certain weapon, ammunition type, or guidance package is worth the cost. Plan ahead for anticipated purchases.
The Mortar should definitely be your first acquisition. Even the best marksman will occasionally let one or two leakers through to the ground, and you need to be able to deal with them. Aimed guidance and HE ammo for the Mortar should be bought as soon as they are available. Then, with the Mortar as a safety net, you can begin to experiment a bit.
When Things Get Hairy:
On the later raids, don't worry about every single trooper. Your ground defenses should be able to cope with quite a few grunts and the occasional special forces trooper. Just keep an eye on them once in a while to check that their ammo supply and defense integrity is holding up. You should concentrate on shooting commanders, special forces, and anything that looks like it will land inside your ground defenses.
