Well, to use the explosion ability, you first need to move the demon close to the target and only use the ability on the next turn. The damage is about 30% of the normal damage. Besides, I usually use the abilities of new demons after removing retaliation or to finish off enemies. Also, they are indispensable when attacking archers, especially low-level ones. However, the fact that you have to upgrade so many things really slows down the development of the castle, and that's why you often don't have enough money.