actually difficulty doesn't matter much. I have A's on everything except homefront, aftermath, and blue on blue. Most of your point go into how many enemies you kill. Mission using the chopper are simple, just kill all targets and don't die. Same with the AC-130 mission. for the fleet mission i used an F-2 and i got an A with that. For the hurricane mission, there's a tricky part because, there isn't much enemies. You should have enhanced missiles and machine gun on your skill for quick kills as you chase Markov. However, when Illich starts to turn around, you turn around and jump the enemies that try to leave the map. Use 4 or 6AAM for this. Then kill any other non-important plane then deal with illich while avoiding damage.
For the last mission, kill every aircraft you see before dealing with the targets. Then when fighting Markov avoid taking damage and save your flares for when his QAAMs come in.Launch was easy since you're in a bomber and that counts toward your score. Just avoid taking too much damage when trying to destroy the ICBM.In Nightmare, just kill everything as fast as you can while avoiding too much damage. For inferno, i used an Su-35 and it seemed to work well, so i got an A on that. Power Play is one of the easiest to get an A in. Just kill everybody once they show up, but be sure to kill all the non-important planes before dealing with the targets. You'll have to fight them anyway since they'll try to kill you. For Siege and Pipeline, use a PAK-FA and use trinity when necessary, like on the fortress in pipeline. Now for lock and load, getting an A depends on hwo many you kill and how much damage you avoid. It will also help for all mission to equip extra armour so you can take more damage. Now, for lock and load best to kill everyone and avoid taking damage especially from the rpgs. the AA guns on the third ship will damage you so try not to take any more damage before then, and after since you have to fight another Hind. Avoid damamge and kill everyone and you should get an A. Hope this helps. Also equip enhanced weapons for quicker kills.
Oh, and as for fleet, when using the F-2, DO NOT engage ASM because you'll take damage while doing so. Make strafing runs on the carrier while moving fast, and be sure your aim your special weapons at different areas to maximize damage. However, use some of your specials for the carrier, and the others on the 2 ships you have to overwelhm. Everything else you can use your missiles.