You could either go with just regenerating it over time so the player has to juggle with their timing, or maybe you could also add that going deeper/falling regenerates some energy as well. So that as long as you progress you wouldn't go out of energy but once you start fighting, you have to manage it more closely.
Another approach could be to create collectibles that regenerate portions of the energy. That could also serve as a motivation for the player to try and reach difficult places in the level