Swap out Rogar as soon as you get a 6th light campaign hero. Prioritize on leveling Roland and Tenada.
When grinding, focus on repeating stages that will give you equipment to equip onto your heroes. You will eventually drop all of the starting heroes in the light campaign. Progression is nowhere near as fast as it is is now than it will be when you run out of energy.
The first thing you need to know is that this game is long term with slow progression.
