an authority on

From LSJ

ἔτυχες εἰς τὴν μάχην ὑπὸ τοῦ στρατηγοῦ πεμφθεὶς → you happened to be sent into the battle by the general

Source

English > Greek (Woodhouse Extra)

ἐπιστήμων, ἔμπειρος