This is a reference to the plagues that the seven angels hold, the ones that complete the wrath of God. Furthermore, the words “ways” in the third line of the song and “acts” in the last line (Revelation 15:4) are synonyms for the word “deeds.” In other words, John hears the conquerors praise God in their song for the plagues that complete his wrath on earth.
3 And they sing the song of Moses, the servant of God, and the song of the Lamb, saying, “Great and amazing are your deeds, O Lord God the Almighty! Just and true are your ways, O King of the nations!