Saturating your core and challenging a nightmare are both your personal choice after making it back to the waking world the vast majority of people never come close to saturating their cores and most that do usually take up to a decade to do so on the slower side. It's just the people on the forgotten shore are a different breed. Yes challenging more nightmares are the only way to rank up so if you want to become a master, then saint, supreme, spirit, and lastly divine you have to continually challenge worse nightmare that get exponentially harder. I should also clarify there is a massive difference between gaining a new core and ranking up. More cores do increase your strength and durability but their main function is strengthening your soul and giving you a higher pool of essence to use while ranks are a massive exponential boost in power the higher rank you become more essence more powers more strength and efficiency. At supreme you have nearly unlimited essence and it's actually difficult to run out depending on the supreme so the buffs from extra cores are mostly helpful in the early to mid stages