I literally just finished this mission. Gotta say it gave me a lot of trouble.
Here's what I did guys in case anyone is still looking for help.
Domon at Level 50. Had the skills Serene Mind, School of Master Asia, and Final Sacrifice.
Knight Gundam with fully maxed out armor and defense and just short of maxing the attack. Had Long Range, Hard Strike, and Pressure Hit (I think that's the name. It broke their guard on a charge attack)
Duo Maxwell. It saved me completely in the end to have him as well. Keep in mind - I had him at Level 5 friendship, which was also very important.m Turtle the partner gauge until the very end for 3 uses (or 4 if you're lucky).
I kind of followed the OP's strategy to a fault. I took the left/top side and made a beeline straight towards the first Fortress. Killed 2 aces by luck as they came streaming out of there, then took the Fortress. After those two aces I made it a point to not get near another one at all. Sprinted towards the Repair Base, snagged that, and then ran over to the Advance Base. Important here to run ASAP to the Advance Base and skip past the Musha Gundams as the second you capture the Repair Base they will start chasing your around the map.
Once I took the Advanced Base, I headed back towards the Main Base and recaptured it, made a beeline for the Missile Base and then worked my way around the map piece of piece. With both the Main Base and the Repair Base working, it gave me a little breathing room so I started to kill the aces. With Serene Mind, it's easy to turtle up the SP and let it build until you come in contact with one and then just start it off with SP attack. Back off a bit, attack, and go straight to CH1 and it hits him with a flame wall. Keep repeating it it keeps them from getting too close.
Once I had the map secured, made my way towards the final area. Knight Gundam got backed into a corner and the Musha's spawned on top of me and my ally. Here is where Duo came in - I turtled the partner gauge the whole game and had 3 times to call him. Pull him down, use a quick CH1 attack and it breaks their guards. From there you can pummel them while Duo is mauling them as well.
Once I had the Musha Gundams out of the way, it was pretty simple to just knock Knight Gundam down really quickly.
I don't know, maybe someone will find this useful. That's exactly what a I did and I'm fairly certain there was a lot of luck involved.