The insect maid Entoma, who was at a level of 51, was almost killed by three members of the Blue Roses, an adamantite ranked team which is the highest rank a team can get.  For a while Tia[A Ninja] and Gagaran[A Warrior] put up a pretty good fight but didn't have the strength between the two of them to win, especially with all the high-ranked equipment she possessed.  But then Evileye[A Vampire] joined the fight with an insecticide spell and powerful magic that quickly turned the tide in the party's favor.  Considering Evileye had pretty much the perfect counters to Entoma she could probably have taken her out even without her teammates help, though with some trouble, mostly because of Entoma's high-end equipment.

Considering we are at most 9 levels above Entoma it is most certainly possible that a well-equipped, high level party would have the means to kill a chapter keeper.  Still, they would have to be at least adamantite in power, or have a lot of high level adventurers, to have a decent chance of winning, otherwise it would be like an under level mob trying to take down a raid boss.

New world entities that could probably easily kill our level 60, or below, characters would be the Dragon Lords, the Elf King, and the God-kin, who are the descendants of previous players and New World residents.  So while most of our characters can easily wipe out most enemies with ease there are still a decent amount of opponents to watch out for, not that you're very likely to run across any of them of course.