For this achievement, you have to gain 3 stars on all 3 difficulty levels. Don't worry, though, scoring 3 stars might actually be easier on Expert difficulty than it is on Normal, and it's not anywhere near hard on Normal! I would write down strategies for each level, but you honestly don't need them.
Here's some general tips:
- Score determines you star rating, and it seems like the limit is 300k points on normal and 700k on expert, but the limit varies from level to level. You can reach these scores by just destroying most enemies in each mission area. You don't need to mop up every single footsoldier, but the more you can kill in a short timespan, the better.
- Time matters, but is not the primary source of points. Just try to finish the level in around 10 minutes, and you should get the default point bonus which will push you past the 3 star mark. On normal, the time bonus is 150k for beating it around 10 minutes (some levels allow 15 minutes). This will come naturally as you play and if you don't waste time; you don't need to haul ass and speedrun it or anything.
- In fact, speedrunning is a bad idea since you won't be destroying as many enemy units that way and might not reach the score needed. Go from objective to objective quickly, but kill most of the enemies before you leave.
- For tough areas, like the beginning of level 5 for example, you can stay away and take your time picking off AA turrets and other dangerous enemies, without losing much time or points.
- Don't worry if you die! There's no penalty for it whatsoever. It doesn't add to your time and doesn't deduct any points. I died upwards of 10 times on the Crazy Khalid fight and still finished with 3 stars.
- You don't need to care much about getting the double score bonus. It's nice and helps, sure, but it's almost never necessary. The only exception is level 11, where you should try to hit large convoys with special missiles as often as you can.
- Always stay moving and spam flares whenever you're fighting something that fires missiles. These parts are the hardest in the game, since missiles take you down very quickly on Expert.
- Level 8 was the only level I had to replay to get 3 stars, and it's because I didn't kill enough of the enemies surrounding the AA turrets at the end. Make sure you do that before you destroy the last AA.
- Pick whatever helicopter you're comfortable with, but I suggest the Kalinin. It's got decent armor, easy to use homing missiles, is available in both Recon and Attack classes and it has the best and most devastating special missile. It's a guaranteed double point bonus if you fire it at a decently large group of enemies. For Supply missions, I recommend the Stormhawk as it's almost the same as the Kalinin, except the special missile requires more careful aim and the homing missiles take some getting used to.
And that's about it. The game might seem intimidating at first since you die very quickly on Expert, but the 3 star ratings are very lenient and not tough to achieve with the above in mind.