A strong angel (εις αγγελος ισχυρος). Here εις = a, just an indefinite article, not "one" as a numeral.

Took up (ηρεν). First aorist active indicative of αιρω.

As it were a great millstone (ως μυλινον μεγαν). Late adjective, in inscriptions, here only in N.T., made of millstone (μυλος, Matthew 18:6; Revelation 18:22), while μυλικος (Luke 17:2) means belonging to a mill. This is not a small millstone turned by women (Matthew 24:41), but one requiring an ass to turn it (Mark 9:42), and so "a great" one.

Cast (εβαλεν). Second aorist active of βαλλω, to hurl.

With a mighty fall (ορμηματ). Instrumental case (manner) of ορμημα, a rush, old word from ορμαω, to rush (Matthew 8:32), here only in N.T.

Shall be cast down (βλεθησετα). Future (first) passive of βαλλω, the same verb (εβαλεν), effective punctiliar future. Like a boulder hurled into the sea.

Shall be found no more at all (ου μη ευρεθη ετ). Double negative with first aorist passive subjunctive of ευρισκω. See Revelation 9:6 for ου μη with the active voice of ευρισκω.