And necromancers struggle to heal troops that don't have your leader with them, until you get some research going. I mostly try to get a healing hero to join me, but that depends on luck. Warlord get a little healing, so do Dreadnoughts but they get it too late to help) then it gets trickier.

not a necromancer, theocrat or archdruid. If you play one of the three races without healing abilities and have a class that doesn't get healing either (i.e. Basically: don't let injured troops fight, and make stacks that all have the same movement rate so the AI won't rush your cavalry ahead on suicidal charges.) (There are some specific tricks to make autocombat work. It's only tricky early in the game, or when you're playing multiplayer and have to use autocombat a lot. Just attack weaker enemies and try to stay out of harm's way. That's pretty much the same for all classes. But that depends on race as much as on class: draconians and orcs and goblins get bonus to healing rate, humans, halflings and dwarves get priests that heal, Elves, Frostlings and Tigrans have to depend on the creation adept sphere (which gives you a healing spell.) or heroes/leaders that can heal or boost the healing rate. The only big difference in the early game is how much access to healing you have.

Rekov: The only thing that I think would be really helpful is some basic strategy on leveling up the different classes of hero.

Theocrat age of wonders 3 tech tree