The mage leader + warrior lord is always the best combo for the campaigns imo, no matter the race. Because eventually they can have good initiative, tankyness and one shot the entire enemy party (minus 2 slotters). OK maybe not for the empire, because there you can get that overpowered artifact that gives the melee dude paralysis on hit.
After mission 3-4 you usually have at least a 10 armor banner + ironskin potion (10 armor) + natural toughtness (20 armor) + at least 10 armor from artifacts on your mage, which allows you to frontline him. Then I like to put 2 fatties on either side (for legion especially this is great as their 2 space units are the best in the game, but the final upgrade is costly), and because of the low hp / high armor on the leader monsters will focus him and he is cheap to heal if you need to - 1 gold per damage regardless of lvl, also you can take a lvl up perk that regens him 15% extra hp per turn.