A strong angel (heis aggelos ischuros). Here heis = a, just an indefinite article, not “one” as a numeral.
Took up (ēren). First aorist active indicative of airō.
As it were a great millstone (hōs mulinon megan). Late adjective, in inscriptions, here only in N.T., made of millstone (mulos, Mat 18:6; Rev 18:22), while mulikos (Luk 17:2) means belonging to a mill. This is not a small millstone turned by women (Mat 24:41), but one requiring an ass to turn it (Mar 9:42), and so “a great” one.
Cast (ebalen). Second aorist active of ballō, to hurl.
With a mighty fall (hormēmati). Instrumental case (manner) of hormēma, a rush, old word from hormaō, to rush (Mat 8:32), here only in N.T.
Shall be cast down (blethēsetai). Future (first) passive of ballō, the same verb (ebalen), effective punctiliar future. Like a boulder hurled into the sea.
Shall be found no more at all (ou mē heurethēi eti). Double negative with first aorist passive subjunctive of heuriskō. See Rev 9:6 for ou mē with the active voice of heuriskō. Already the old Babylon was a desert waste (Strabo, XVI. 1073).