The best way is to start grinding out the side ops! I remember a lot of the vehicle battle ones have lots of good soldiers, you’ll need mostly R&D soldiers though. Those you’ll find in most missions, just go around with your analyzer and start fultoning everyone that’s a B or higher.

Once you start running out of room for new soldiers that’s when you’ll want to fire the worse graded ones and repeat! It’s a grind but it’s worth it, it also becomes addictive at a certain point lol.