I have 1 last achievement to get and it didn't seem like it would be that hard since I already got S rank without even trying on all the high payout missions when I was grinding cash for all the parts.
However, a few seem to be completely arbitrary and not simply about speed and efficiency. Or maybe I am just not fast enough? I have to actually use a stop watch because the game doesn't actually tell you the mission time...
Anyway I have literally 2 left that I can't get above A rank on and I don't know why.
First is Stop The Data Breach. I zip through the level as fast as I can destroying the hacking nodes, and even take out the tetrapod in the hallway while waiting for the dialogue to continue and give me access to the last door leading to Iguazu. I know from watching a video of someone getting S rank that the damage and ammo costs I end up with are good (even lower) so it's gotta be the time it takes to do the final fight. But that dumbass Iguazu heals like 4 times every time he's down to nothing which makes it take way longer than it should. I can kill him faster but that would cost more in the ammo cost, assuming it even hits because the weapons that would allow near instant death are also super slow and he constantly dodged all over the place.
The other is Destroy the Refuelling base. I've zipped straight to the main objective and killed the ACs at the end in under 2 minutes and got A rank. I've gone and blown up all the fuel dumps then the main objective and killed the ACs in just a few seconds more; still A. The fuck does it expect me to do? Finish in 30 seconds? I can use the fastest possible mech build and it still takes like a minute just to reach the objective going in as straight a line to it as you can.
The lack of clear objectives listed for these arbitrary requirements is driving me crazy. I've looked for answers online, of course, but every answer is different. Some people got S rank doing this, others got it doing something else. Neither method worked for me, so it just feels like random.
Hey, I've actually done that! It was almost a year ago now, so I can't remember my exact strats for those missions, but I might be able to help.
First of all, those two missions are brutal--I had to retry them a lot before I got the S. It seems like you've got the right basic idea for both: move fast and play the objective above all else.
For builds, I had the most success running Zimmerman in the right hand and laser lance + pile bunker on the left hand/shoulder. You can swap lance/bunker to basically always have a melee available to one shot any MTs that are in your optimal path. For the real fights, building up poise damage with Zimmerman and then staggering with lance before finishing with charged bunker is an insanely fast kill that only costs Zimmerman ammo. It takes some skill and a little luck to land it on Iguazu (he's one slippery bastard) but if you can lance him into a corner then he's toast. The same basic principle applies to the refueling base fight, but you have to do it twice. The biggest thing to know is how much poise damage you need to build up before lance will stagger --it's crucial that the lance induces stagger to set up the bunker.
Good luck!
Edit: I see you got it--congrats! That's a tough achievement.