as is said above already, things depend on what mission you play, and your play style.
most its events let you bring two lists to choose from for every battle. Since you have (and this is a very big generalisation) two kinds of missions, push buttons (the once you need specialists for) and kill the enemy (preferably till his troops die from it).
To learn the game, the second type of mission is a bit less complicated (in my humble opinion) and the list building is simpler.
play as much as you can and try lots of things proxy models is the magic word (s)