I think most RPGs are a great candidate for this question. If a villain suddenly knows about leveling and gear stats, his main goal would become to level up as much as possible, potentially starting a whole genocide to farm XP
Also how would politics and society change? Do you tax level ups? Make new laws that keep certain builds illegal? Would you be able to join the Imperial Army after completing the Dark Brotherhood in Skyrim?
If I were a Souls boss I’d probably just stop fighting. If my opponent is immortal and keeps coming back to fight me and learns more every time, I’m just leaving.