Warhammer Age of Sigmar: Storm Ground

It takes 1000 XP to get to 3rd level with a base unit. 500 for level 2 and 500 for level 3. The humans have an archer and tank base units. The archer units are really good at getting kills, especially with some good gear. If you can keep them alive, they will level up fast. For the tanks, you'll probably need to tribute gear for XP. Wait until you have 1000 XP in Tribute and spend it at once.