Joseph Smith emended was to were here (with were spelled as wer). Perhaps he was influenced by the preceding were (“those that were with him fell again to the earth”). Since the subject for the original was is the singular noun phrase “their astonishment”, this initial emendation of was to were was never implemented in the actual 1837 edition. See verse 16 for a similar aborted emendation by Joseph Smith (in that case, an inserted not).
Summary: Maintain in Mosiah 27:18 the singular was in the clause “for great was their astonishment” (the reading of the earliest textual sources).