Your first class will automatically be equipped when you unlock it from the skills tree. To change your class once you’ve unlocked new ones, go into the ‘tactics’ section when you camp at a bivouac.
Ranking up your classes is simply a matter of completing the objectives through each stage. The game doesn’t exactly make it clear that there are even class objectives, so it’s easy to miss.
To see your current rank, and the objectives needed to rank up, go to your load out screen. Your class emblem is in the upper right - hovering your selector over it will show you the objectives, which can be pinned to your hud just like the missions.
I’d go with Panther for the fastest way to get this trophy. Your hunter drone kills count towards many of the stealth objectives and much of the rest come naturally anyway.
When it comes to the SMG ‘kill x number of enemies without reloading or switching’ - change to single fire mode (
whilst aiming down the sights) go for a headshot, then move, then shoot. Pick them off one by one and you’ll rack up the kills at the same time.
I’m dealing with the other classes now, if anyone else has tips to help out there, let me know and I’ll make sure it gets mentioned.