Save before battles and reload if you don't like the outcome When hiring a general, you can close the window and reopen it to get a new selection Whenever available for your general at lvl up, pick Master of Maneuver to have more units in your army (2 ranged and 3 melee are a good mix) Let them tank dmg, heal them with your general (preferably a caster with cure spell, as someone already mentioned) Clerics are tanks with acceptable dmg, they're your third unit for your doomstack.
When you get the choice to upgrade, get Marksmen With regards to army management, my requests for help on reddit yielded this particularly comprehensive reply: