for the first few play-cycles there's really no point trying to kill any demon lord by brute force, it's a waste of time unless you cheat
just go with the easiest option and get to as many endings as possible for cycle-bonus, you need to clear ALL endings form 'Part one' to be able to start 'Part two' of the story anyway and I think normally you need to play at least 3 to 4 cycles to get all endings.
start trying to kill demon lords by brute force only after you have at least 8 or so cycle-bonuses
also pay attention to your difficulty level, need to keep it well managed for easier progress
major route splits (Rance castle defense) happens at turn 8:
1. if 1 or more kingdom falls to demon king army, Rance Castle is relocated to JAPAN
2. if no kingdom falls, but Shangri-La is not captured, Rance Castle is relocated into the mountains of Leazas-Helman boarder
3. if no kingdom falls, and Shangri-La is captured, and the red-eye is defeated during the Rance Castle defense, the castle becomes gains floating capability; if fights against the red-eye are lost, the outcome is the same as 2.
basically you can only get to A type of clear (true historical routes) after achieving floating Rance Castle except for the sea route.