Here the RLDS 1874 edition introduced the plural powers into the RLDS text. However, elsewhere in the text, there are 20 occurrences of the singular phrase great power, but none of great powers. Nearby, there is one other example in the text of his great power, in Alma 26:16: “who can say too much of his great power”. The plural is, of course, theoretically possible here in Alma 18:2; consider the occurrence of powers in Mormon 4:17: “and in this year they did come down against the Nephites with all their powers”. For each instance of power(s), the critical text will follow the earliest textual sources, thus the singular power here in Alma 18:2.
Summary: Accept in Alma 18:2 the singular power when preceded by great, as with 20 other instances of great power in the text.