In this passage, the original text reads “a wonderful great army”. Theoretically, the word wonderful can be interpreted as either an adjective or an adverb. In the adjective case, wonderful modifies army and means that the army caused wonder. In the adverb case, wonderful modifies great and means that the size of the army caused wonder. The editors for the 1920 LDS edition decided that wonderful was an adverb in this passage, and thus they grammatically emended wonderful to wonderfully. Elsewhere in the text, there are only two examples of the attributive adjective wonderful (that is, where wonderful acts as a premodifier):
As discussed under Alma 42:15, there are many examples in the original text where adverbs lack their expected -ly ending. The phrase wonderful great was quite frequent in Early Modern English and meant ‘wonderfully great’, as in 2 Chronicles 2:9: “for the house which I am about to build shall be wonderful great”. Here are some additional examples (cited with accidentals regularized) of wonderful great as found on Literature Online ; in these examples, wonderful is clearly adverbial:
Given the historical usage, the 1920 interpretation of wonderful in Alma 51:11 as an adverb modifying great is most likely correct. The critical text will, of course, restore the original wonderful here in Alma 51:11 no matter whether wonderful is an adverb or an adjective.
Summary: Restore in Alma 51:11 the original wonderful without the -ly ending (“a wonderful great army”); this instance of wonderful great has the expected meaning ‘wonderfully great’.