I think they won against the supreme king because they ALL fought together, and I think it was mentioned before that the generations grow stronger should their parents also be gifted.
The witches also mentioned that their strongest is the radiant one (forgot her name)
They mentioned one of the strongest that wasn't present, also I think you should consider their compatibility
actually in ch.37, it mentioned that with each passing generation, the royal blood gets thinner and hence the powers get weaker.
however, it's true that the strongest witch wasn't present this time, so that makes sense
Oh, okay thx for reminding me /s
The power scaling is not consistent. So the coven of witches managed to bring the Supreme King to the brink of death, but somehow they're having trouble fighting Arkos/Altus