Hello! Thank you so much :D Super happy you enjoyed the game.
For Serin's Bad Ending, you'll have to go on Serin's Romance Route and increase his affection as much as possible (can be easily achieved with Beauty & Charisma). For Week 60, keep Serin's Light AND Wood magic under 170! And tadaaa :D Bad Ending ahoy!
(If you have a save file from his romance route from before maxing out his stats, you can probably use it.)