In light of the previous post about creating dramatic endings, one question that could be asked is: what if you can't think of a good ending?
That could be what was in Bram Stoker's mind when he ended Dracula how he did, even though I seriously doubt he didn't really know how to end it.
One answer that comes to mind is: it doesn't have to end . . . with that one novel. Could it be a sequel or a trilogy or even a long series? Then your monster wouldn't have to die or end, it could live and grow stronger. Need an example? Lord Voldemort in J. K. Rowling's Harry Potter is one. He keeps growing more powerful with each book.
Another answer would be to write three or four alternative endings, have a group of trusted friends read them (ones that can give you an honest opinion, not a sure-this-is-good-but-only-because-we're-friends-and-I-can't-tell-you-the-truth). You need the truth here.
If you can't even do that, skip the ending (for now) and go on to something else. Then, after a week or a month, come back to it. If you still can't think of one, then maybe your story isn't as good as it should be. And if that's the case, write another story.
A final word of advice on this could be: what books or movies are similar to your story? How did they end?