Here the 1905 LDS edition accidentally dropped the indefinite article a in the phrase “with a great slaughter”. This omission has been maintained up through the present LDS edition (1981). Elsewhere in the Book of Mormon, phrases involving great and slaughter all have an article, either the indefinite a /an (six times) or the definite the (four times):
In none of these cases do we find slaughter without a determiner. The critical text will restore the indefinite article a here in Alma 2:18 (“with a great slaughter”).
Summary: Restore the indefinite article a before great slaughter in Alma 2:18.