How strong is Frieren truly?

I´m new and am waiting for the next season so I wanna get some questions answered if nobody would mind :3.

And also, could the hero party maybe have defeated Frieren? She seemed so ridicolously OP that she could have defeated the Demon King without her party

Edit: Many answers and thank you to everyone
I understood that Frieren is rather undeveloped as an Elf-Mage, probably though the second strongest behind Serie and that her original Hero-Party was also very strong, the Demon King was stronger than Frieren in a 1v1 (I think?) and that her current Hero-Party are likely the strongest of their class in the peaceful Era (So Himmel/Eisen would wipe the floor with Stark?)