The syntax for this clause seems unusual and suggests the possibility that against is an error for again. The original manuscript is extant here, and the reading is clearly against, not again. The against is definitely intended since Oliver Cowdery initially wrote again, lifted his pen, and then wrote inline the st (with slightly weaker ink flow).
Elsewhere in the text there are two other occurrences of deny occurring with again but never with against. These two occurrences, in fact, occur in this same narrative:
Korihor had previously denied the existence of God (in verses 28 and 38) and that there would be a Christ (in verses 12–15 and 26), so the use of again in these two passages is perfectly expected. However, here in Alma 30:45, Korihor has not yet denied the evidence from God’s creation that Alma has just mentioned:
So contextually, an emendation to again in verse 45 appears to be inappropriate.
There are a total of 83 occurrences of the verb deny in the Book of Mormon. In 77 cases, deny either takes a direct object or is in the passive (where the subject is the semantic equivalent of the direct object). Besides this case in Alma 30:45, there are five cases where deny takes no direct object (although a general one is implied):
Thus Alma 30:45 can be interpreted as one more of these cases where no direct object follows deny. But it does seem a little unusual (yet possible, of course) to use a prepositional phrase to complete the verb deny (“and yet will ye deny against all these witnesses”). However, since the against was written very deliberately in the original manuscript, the critical text will maintain this usage. Korihor has not yet denied these witnesses from nature, so again would not really be appropriate. For another example where the question arises of whether again and against have been mixed up in the text, see the discussion under Alma 14:20.
Summary: Maintain in Alma 30:45 the original against (“and yet will ye deny against all these witnesses”); the use of against here is probably not a mistake for again, although “deny again” occurs two times in this chapter and “deny against” occurs nowhere else in the Book of Mormon; again does not really work here since Korihor has not yet denied the evidence from God’s creation that Alma has just mentioned in verse 44; moreover, the careful way that against was written in the original manuscript strongly argues that against was indeed intended.