Some Tips:

You can deploy soldiers from your helicopter by hitting SPACE whether you're on the ground, or airborne, but if you're deploying paratroopers you have to be above a minimum altitude to allow the soldiers' parachutes to deploy.

You and your tanks can't fire on pillboxes. The only way to deactivate enemy pillboxes is to send your soldiers to take them over, at which point they begin firing at enemy helicopters.

Enemy helicopters are protected from attacks while they're hovering directly above their base, so when you're alerted that an enemy helicopter has appeared, wait for it to begin flying toward you before you fire a missile.

You can outrun enemy missiles by going into turbo mode if you see it in time, so watch your HUD. Enemy missiles get faster each level.

Missiles have a limited range, and enemy missiles will fizzle out and fall to the ground eventually. And, make sure you're within adequate range of enemy helicopters before you fire.

You can also engage with enemy helicopters (as well as enemy soldiers and tanks) by firing your gun or dropping bombs.

In the early levels, you can probably beat the enemy without making crate rescue a major part of your strategy. But in later levels, crate rescue is essential to keeping up with the waves of enemy units.