Welcome to the site Slayer_01!
I am familiar with Buffy and the sequel with Angel. Loved both the series despite having significant initial reservations against the entire concept.
In generationem et generationem femina eligitur. Ea sola contra
hirudines, daemones et alias copias tenebrae stat. Ea interfectrix
Here's my take on your translation, which I think gives a good starting point.
I thought some of this would be easy as I vaguely remembered Biblical passages I had once sung as a kid in both English and Latin. I was wrong, but found stuff that interests me nonetheless.
With a quick browse through the Vulgate, I found "in generationem et generationem" in Psalm 118/119:90 (This is one of those psalm numbers that differ from Bible/language to Bible/language) but the meaning appears to be more like "to all generations," rather than down from generation to generation. I checked the Hebrew (לְדֹ֣ר וָ֭דֹר) and the Greek (εἰς γενεὰν καὶ γενεὰν), since those were Jerome's sources for the Vulgate, and the constructions are parallel to the Latin for whatever that is worth.
With some further browsing, I found Luke 1:50 (a progenie in progenies), which seems to have more of a feel of "from generation to generation" in context; however, the underlying Greek seems to be εἰς γενεὰς καὶ γενεὰς, which is almost the same construction as in Psalm 118/119:90.
In Psalm 9:27/10:6 (the numbering again varies from Bible/language to Bible/language) we have a generatione in generationem, where the Greek has ἀπὸ γενεᾶς εἰς γενεὰν and the Hebrew again has לְדֹ֣ר וָ֭דֹר.
It seems then that to translate "down through the generations," either a progenie in progenies or a generatione in generationem would be appropriate, but then is that actually what the original meaning of the English is or implies?
As I recall, the idea is that a vampire slayer is borne in each generation, so that the world is never without one. I think we need a new approach to the translation then, since I don't think this concept appears in the Vulgate anywhere.
In looking up hirudines in Wiktionary, I see that it means "leeches," but also found sanguisūga listed as a synonym. The roots of the latter show that it means "bloodsucker," and so I think it is a better choice for "vampire." Wiktionary actually lists "vampire" as one of its meanings, but I did not chase down whatever reference they may have had for that.
As for some of the other words in your translation, I think the genitive singular tenebrae is okay, but Jerome uses the plural in Genesis 1:2. The Septuagint has singular σκότος, and the Hebrew has singular חֹ֖שֶׁךְ. I'm guessing that the plural sounds more specific to the context and maybe differentiates "darkness" from the idea of mere "shadow."
I think that "stat" should be in some type of future tense to match the English, if you are going to use it, rather than in the present tense.
"Interfectrix" seems to have been used as "murderess"; however, "interfector" seems to have also been used as "slayer." I like your choice based on this, and don't think I would have found this solution.
Building on your work, here is the translation I would offer:
In quāque generātiōne gignitur ēlēcta. Quae sōla est sanguisūgīs,
daemoniīs, et aliīs tenebrārum cōpiīs obstātūra. Ea est illa interfectrîx.
Here is my reasoning, right or wrong, for some of the changes I made. I used gignitur, rather than est, because est just seems so bland. I think you need illa to get the meaning of uniqueness across and the implied reference back to what was just described about the ēlēcta. I think it is important to the concept of the series that we establish the term "chosen one" and don't just use a verb form to convey it. That is why I used ēlēcta, rather then ēligitur.
I think obstātūra est may be closer in meaning to what is conveyed by the English future in this context than the simple obstābit. Besides, that choice allowed me to use the word order of est in a way to highlight sōla.
I also have made other word order changes that accord with my sense of how Latin would match the English pragmatics. For instance, I think that the hyperbaton in aliīs tenebrārum cōpiīs conveys that this is a unified concept and not an ad hoc collocation of a noun phrase and a genitive.