Endless action is the same as Action levels but now endless, of course. You can choose one of your units and the battle begins. Most used units are the Lich and the Reap. This is easy but gets harder so it is reccomended to use a Lich. Necro and Vamp are ok, but the lich can kill multiple at once. Also the tier 1 or 2 Skully (if you constantly use his special), the Imp, the Troll and the Skully Archer are good.