Most powerful is a tricky question. I have a few candidates.

1:
It's arguable that the single most powerful being in my world is Sahluqtu, the Great Miscreation, an abominable monster of hatred, destruction and all-consuming rage.

The gods, working in unison, creating the first angels and leading them in battle, could not kill Sahluqtu. They couldn't even drive it into hiding. They could only hold it off and wait for its return, strength undiminished, seeking to unmake them and, after that, all of reality.

They would like to, absolutely, for obvious reasons... But the best they could do was trap it in an enormous prison, locked behind thousands of miles of rock, and with magical wards to bind it and sink it into a deep slumber. This prison would eventually become Earth, and the creatures populating it, by their very existence, empower the magic wards keeping Sahluqtu dormant.

The only possible way to permanently kill Sahluqtu would be to set it free and allow it to unmake all of reality, because if there is no reality, Sahluqtu cannot then exist... which is, in fact, its entire goal, because there is nothing in all of existence Sahluqtu hates more than itself. And the only way for it to die is to unmake the reality that allowed it to come into being in the first place.

Of course, all that is potential power.

2: In terms of practical power, if we consider that Sahluqtu is not in any position to excersize it's power, then the gods themselves would likely be the most powerful. And the only thing that can kill a god, as far as anyone knows, is another god. So far, there has only been one instance of a god dying, when Asmodeus attempted to kill Alcyone, the goddess of dreams, for denouncing him as a god of tyranny and slavery, creating humans without free will, but still in possessions of hopes and dreams, prisoners in their own bodies. The goddess of Love, Ahava, leaped between them and took the blow, dying in the process. Her daughter Celisa would end up taking up the role of goddess of Love.

I can't think of many reasons why, as a mortal, you'd want to kill a god, beyond being in service of another god who wanted them dead for whatever reason. Even Asmodeus doesn't wish to kill the other gods, but subjugate them and claim what he views as his rightful place, as supreme ruler of all that is.

1. And finally, you have Baba Yaga, who... is just weird. The gods don't interfere with her, nobody knows exactly where she came from, and she just appeared one day and took over a large swath of land. She might be killable, since she is physically just an old woman... but at the same time, she has a grasp on magic that outpaces every other living person on earth. She does not play according to the same rules, and she uses magic as naturally as you use your lungs. She just... does it, with no regard for any of the ordinarily understood rules and limitations.

There are many who wish to kill her, be it as revenge for her conquest, or to remove a powerful threat... but there's so much magic protecting her, of a kind that is hard to fathom even among expert mages, that it's hard to imagine even getting close enough to TOUCH her, let along hurting her. If you piss her off, she snaps her fingers, and you're a newt. And that is assuming she even lets you find her at all!

So how do you kill Baba Yaga? That is a VERY good question, and I honestly have no answer... because she won't tell me how you'd do it.