All "How do I unlock X" questions have the same simple answer.

[Zizel]

If your wondering how to unlock a certain Unit, Pilot. Skill or remove certain restrictions, the answer is always going to be the same.

S Rank Story Missions.

If you S rank all story missions you will eventually unlock everything, all units, all Pilots, all restrictions removed, all skills. Just focus on S ranking the story missions and you'll 100% the game.

[Charsiew]

for tuning limit release, just need 10 mission to S rank after complete all faction mission. Trying to S rank the other mission on zaku clyne and battle destiny will be challenging, but i know a way to cheese.

[Outsider]

S rank itself isn't always straightforward.
Sometimes you don't need to kill all the aces, sometimes you don't even need to destroy the most obvious target.

Good example is CE73 Phase 7 which I spent quite a lot of time on trying to S rank, until I found this post
https://gamefaqs.gamespot.com/boards/643156-kidou-senshi-gundam-seed-battle-destiny/63438169

Originally posted by NeoSoftware :

I didn't even have to kill Kira for this mission.

For the first part, I killed Stella, then Sting and Auel afterwards. Shooting Cagalli will make Waltfeld launch in his custom Murasame and he can be killed in this part. Archangel and everyone else will eventually leave.

For the second part, I head straight for Cagalli, killing her. Next, I destroy the Archangel. The Freedom starts chasing after me but I fly back to the Orb ships and sink them, without even killing Kira, Sting or Auel.

Got my S rank and Saviour to boot. I think time is a big factor in this mission. You could kill all the aces in the second part, but if you take too much time doing so, the highest rank you'll end up with is A. Oddly enough, Cagalli and the Archangel were enough to earn me that S rank.

You'd think you'll need to whack Kira and the Druggies, but nah. It's more important to keep your NPCs alive and be real quick.