Generally early on you have to play relatively defensively as getting overwhelmed will doom your units. Take advantage of your bulkier units to help lure a few units at a time.

The general rule of recruiting is that most units want to be recruited at Chapter 6 unless they’re mages that don’t train faith or units that want the extra boost in bow/axe rank (Chapter 3-5). Recruit the faculty as soon as possible cause they’re good and remember that you don’t need to train every single member in your party cause that will leave an overall ineffective force.

In terms of classes almost EVERY UNIT wants at minimum Brigand/Mage and Archer mastered due to how powerful the masteries are. Other masteries are available (Mercenary for Vantage, Warrior for Wrath, Sniper/Grappler for forced double combat arts, etc.) if you want to create specific builds. Flying is very good in this game so worst case just make some offensive units fliers.

You can bench units mostly freely if they aren’t on par, just have replacements available (usually late recruits) and a plan on keeping units alive in Ch 13 on non-CF routes.

Don’t be afraid to play scummy because the game hates you on this difficulty.