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.