There are three requirements that need to be met to ensure an overall ranking of A.
- Difficulty needs to be set to at least Hard
- Need to get A rank on all 5 rescue missions
- Continue less than 5 times
Time, enemies killed, and number of saves are not calculated into the ranking, so feel free to save as often and avoid as many enemies as you would like. Though keep in mind that ignoring enemies will mean your level will likely remain low.
Hard difficulty is completely manageable given a little practice and understanding of the combat. Burst shot and rush blade attacks are normally your best bet when it comes to finishing enemies off quickly and safely. The Burst Shot also works well on all bosses except for Neith, who I would recommend you spam Gauntlet (2nd fight) and Comet (3rd fight) on. If you are having difficulty at any point, you can always farm previous areas to level up (max level is 8).
Perfecting the rescue missions is probably the hardest part to this achievement, as not only do you need to save 100% of the survivors but you also need to make sure that no more than 10% of the buildings are destroyed. The basic tactic is to trigger all the enemies at once and draw them to an area where there are little to no buildings. You'll also need to keep an eye out for new squads and catch their attention as they spawn, otherwise they'll go about destroying the area while you are still fighting the original groups. The second mission is probably the trickiest, as it is also the area where you fight Tempest, so it is likely that at least several of the buildings are already destroyed. For more in-depth strategies on how to ace these missions and mitigate damage caused by Tempest check out the solutions for the corresponding achievement.
Finally you need to make sure to not continue more than 4 times. You might have 5 continues, but I'm not sure, so until someone confirms otherwise I wouldn't test it. This is easily avoidable, as all you need to do is make sure to save often and never continue when you die. If you die, return to the main menu and just load up your last save. You can save as often as you like, since the number of saves does not affect your overall ranking.