Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
εχθρεύσω I will be an enemy LXX.Exod.23.22
εχθρεύσουσιν they shall be an enemy LXX.Num.33.55

Liddell-Scott-Jones

ἐχθρεύω (echthreuō) be an enemy G:V

ἐχθρεύω-εύω,

to be at enmity with, τινι [LXX+5th c.BC+]

LSJ Scripture Refs: LXX.Exod.23.22