Here's a good guide that I found that helped me through the missions. Most of them I didn't have a problem, but there were a couple that I had to do a number of times and these strategies helped.
Some notes to make the game easier:
1) Don't bother using ATA missles against enemy helos unless you're within close range (.5 or less). Just make sure you lead the target so the missile doesn't have far to turn. They will always drop flares and you will miss. Drones however are open season...
2) Helo Pads. You'll see these marked as "H" on your map, if you land on these pads you will rearm and refuel. Once you land on the pad, it takes about 11 seconds to activate. One VERY important thing to note. If you land to rearm, this BYPASSES the rearm timer on your missles. So for instance, you can launch all 8 of your hellfires, land, and 20 seconds later have another 8 to shoot. This comes in handy BIGTIME on certain missions. Now they do have a cooldown timer, so after you reload you'll need about 20-30 seconds before you can do it again.
3) Crashing - Don't be afraid to crash, I was at first and then I realized that yes you only get 3 retries. BUT, it seems the retries reset themselves after you complete each part of the mission. So for example, in one missions you get 3 retries until you complete the first objective, then you get another 3 tries for the second objective, etc.
Mission 1: Takeoff (Tutorial)
Simply follow the on-screen instructions to familiarize yourself with the controls, weapons and combat.
Mission 2: The Sky is Burning
Take off and eliminate the enemy forces approaching from the south-east. Use rockets.
Then, eliminate the enemies located north-west. Use rockets.
Afterwards, eliminate all remaining enemies located south-east (the fleeing ones as well).
Now, fly south-west towards the waypoint.
Eliminate the enemies along the way using rockets.
Mission 3: Lord of War
Take off and fly west to the marked location.
Then, eliminate the enemies located north by using rockets.
Fly north to the next village and eliminate all enemy forces using rockets.
Fly to the village located west and once again destroy all enemies. Use missiles first and then rockets.
Fly east and destroy the convoy using rockets.
After the short cutscene, take some distance to the crashed helicopter and enter FLIR mode. Shoot all enemies using the machine gun and rockets. Do not use rockets close to the crashed helicopter however.
Escort the rescue helicopter to the landing zone.
Eliminate all enemy forces shooting at the rescue helicopter along the way.
Finally, land at the landing zone (marked with an H on your map) to complete the mission.
Mission 4: The River Deep
Take off and fly north towards the waypoint.
Eliminate all enemy forces using both rockets and missiles. Always keep moving and fly at an altitude of between 250 and 400. Focus on the enemies launching rockets first.
Then, eliminate the fleeing enemies. Use rockets.
Attack the following enemies and finally retrieve to the landing zone located south. ?#Land the helicopter at the landing zone to complete the mission.
Mission 5: Whiteout
Fly along the valley, eliminate the enemies ahead and continue to the waypoint.
Destroy the following enemy forces and the enemy camp. Use rockets.
Proceed west to the next waypoint and eliminate the enemies along the way.
Continue south through the valley, destroy the enemy forces using rockets and the enemy camp using missiles.
Proceed to the next waypoint, eliminate the enemies along the way using rockets and destroy the enemy camp using missiles.
Fly along the valley to the next waypoint and destroy the bridge. Eliminate all enemy forces along the way.
Fly back north through the valley and destroy the remaining enemy convoys.
Then, fly south towards the landing zone and land the helicopter to complete the mission.
Mission 6: Sharks in the Water (The hardest in the game IMO)
Fly straight ahead and eliminate the pirates in the bay. Use rockets.
Try and stay low throughout the whole mission. It is easier to aim.
Then, fly south and protect the four blue marked ships.
Eliminate the enemies approaching from the south. Use rockets.
Then, turn around and eliminate the enemies approaching from the west. Use rockets.
Now, fly towards the enemy approaching from the west and eliminate him. Use rockets.
Quickly turn around and aim for the enemies approaching from the east followed by the ones approaching from the north. Use rockets and stay close to the blue ships.
Turn around and use all your missiles for the enemies approaching from the west.
Then, fly towards the enemies approaching from the south and south-east. Use rockets.
Eliminate all remaining enemies around the blue ships.
Then, fly towards the next target and eliminate all pirates around the conquered ship. Be careful to not destroy the conquered ship.
Afterwards, simply stay close to the ship to complete the mission.
Mission 7: Holidays in Hell
Switch to FLIR mode and search for survivors (little white marks) near the sinking ship.
Mark them for rescue.
Then, fly to the marked location at the coast and eliminate all enemy forces. Use missiles at first and eliminate the remaining enemies with rockets. Always keep moving.
Land at a landing zone to rearm.
Fly to the next location through the canyon and destroy the bridge and all enemy forces around it. Use both missiles and rockets.
Land at a landing zone to rearm.
Now, fly to the next location and once again eliminate all enemy forces. Use missiles first and then rockets. Keep moving and evade the enemy rockets.
Mission 8: Over and Out
Fly along the valley ahead to the power plant. Eliminate the enemies along the way.
When you reach the power plant, use as many missiles as possible and then use rockets for the remaining targets.
Next, you need to destroy the upcoming convoy. Fly towards it and use rockets for the vehicles further back. Once the vehicles get too close to the dam use machine gun fire since you must not damage the dam.
To complete the mission destroy the two communication towers. It does not matter which one you destroy first. Simply, fly to the locations and use rockets to destroy the towers and enemy forces.
Mission 9: The Gang’s All Here
Take off and eliminate the enemies ahead using rockets.
Then, eliminate all enemies around the landing zone. Use missiles at first, then rockets.
Auto hover and enter FLIR mode. Shoot all remaining enemies.
You need to pave the way for the infantry troops. In FLIR mode eliminate all enemies ahead of them. Enemies are hiding between the trees. Destroy the tree using rockets to discover them.
Fly to the next waypoint and eliminate all enemy forces. Use missiles first, then rockets and always keep moving.
Mission 10: On the X
Proceed to the waypoint and eliminate the enemy forces ahead. Use missiles first, then rockets. Always, keep moving to evade enemy missiles.
Fly to the next waypoint and destroy the enemy helicopters by using ATAs, rockets and machine gun fire. Stay close to your targets and evade the enemy missiles.
Mission 11: Sunshine Units
Fly through the valley and stay close to the ground.
When you reach the village, destroy the vehicles trying to flee.
Fly very low over the village and let your gunner destroy enemy vehicles. Use rockets as well, but be careful not to hit any blue vehicles.
Escort the vehicles to the village.
Switch between FLIR view and normal view to discover enemies and keep moving to evade their rockets.
Eliminate all enemies along the way. Use rockets and machine gun fire.
Mission 12: A Diamond in the Rough
Fly through the valley to the waypoint. Stay low to not be detected.
When you reach the enemy base, stop/autohover and mark six targets for an air attack.
Take out the remaining targets with missiles.
Then, proceed to the next waypoint.
Now, escort the vehicles to the next location.
Fly ahead and eliminate the enemy forces. Use both rockets and missiles. Enemies are located left and right of the valley and further along the way. They are all marked on your display.
When the vehicles reach their location, you need to fight to enemy helicopters. Use rockets, ATAs and machine gun fire. Constantly move to evade their rockets. Use your ATAs once you are close to the enemy helicopters and always fire a bunch of rockets.
Afterwards, follow the marked helicopter to complete the mission.
Mission 13: City on Fire
Fly towards the city.
Then attack the approaching enemies.
Prevent them from crossing the bridges that lead over the river from south to north. Use missiles for the tanks and rockets for the other vehicles.
Once the objective of protecting the bridges is complete you need to destroy them. Fly close to them and shoot a bunch of rockets at the bridges.
You now need to protect the city hall located in the northern part of the city for 5 minutes.
Fly back and forth across city hall and destroy the approaching enemy forces. Use missiles for the tanks and rockets for the other vehicles. There is a helipad on top of the main building if you need to rearm your missiles. I would use hellfires to kill the two tanks leading each column, try to kill as many soft targets with the cannons, and then land to reload the hellfires.
Once the air attack starts, simply eliminate all remaining enemies to complete the mission.
Mission 14: Rain in the Desert
Fly towards the waypoint.
Then, fly all along the enemy convoy and destroy as many vehicles as possible using rockets. Do not worry if you do not destroy all targets.
When you reach the beginning of the convoy, turn around and eliminate all remaining vehicles. Use missiles for the tanks and rockets for the other vehicles.
Once you destroyed the convoy, you need to destroy an enemy helicopter.
Get close to it and use rockets, machine gun fire and ATAs to destroy it. Always keep moving and evade its attacks.
Afterwards, proceed to the factory and eliminate all enemies around it. Use all your missiles first and then rockets.
Mission 15: A Damn Shame
Get to the waypoint. Fly close to the ground to have the mountains as cover. ?#Fly left when the valley splits into two paths ahead.
Once you reach the waypoint you will face a lot of enemy helicopters.
Focus on one after the other and use rockets, machine gun fire and ATAs to destroy them. Always keep moving and evade missiles and rockets.
When you destroyed all helicopters, you need to destroy the bomber.
Get very close to it and use ATAs to destroy it. Use missiles to destroy the enemy forces on the ground. However, focus on the bomber and do not get too caught up fighting the ground forces.
Mission 16: Turkey Shoot
Fly towards the waypoint and shoot the following group of aircrafts using ATAs first and then rockets.
Next, you will face some enemy helicopters. Evade their attacks and use rockets to destroy them.
Fly towards the next waypoint along the river and eliminate the boats on the lake. Use rockets.
Then, you need to eliminate all enemy forces at the base located north from the lake.
Fly to the landing zone south of the battlefield. From there, launch missiles at targets on the battlefield and land to rearm your helicopter.
Repeat that process until all targets are destroyed.
Now, rearm one more time and shoot missiles at the escaping convoy.
Follow the convoy and destroy it. Evade the enemy missiles.
When you reach the next base and have destroyed the convoy, you will face enemy helicopters. Do not try and destroy all of them. You only need to destroy the one trying to escape south-east. Get close to it and fire everything you have at it.
Note: this last part is not easy, but if you focus on the escaping helicopter you will get it eventually.
Mission 17: Ship of Fools
Fly towards the waypoint.
Eliminate the enemy forces at the oil platforms. Focus on the boats first and use rockets.
Then, destroy helicopters using ATAs and rockets.
Now, get in position (autohover) south of where the rescue helicopter lands and enter FLIR mode.
Shoot all enemy targets on the left and right platform. You can use rockets for the right platform. You may fly around the left platform to be abel to eliminate some enemies.
Afterwards, destroy the enemy boats west of the platforms. Ignore the enemy forces on land and focus on the boats. Use rockets and missiles.
Finally, you need to destroy one more enemy boat. Missiles will be ineffective for this one. Get close to it and fire as many rockets as possible at it. Once, you destroyed it you have completed Apache: Air Assault.
I found these at this site, credit goes to them but I have added my own comments:http://www.mahalo.com/apache-air-assault-walkthrough/