To execute judgment (poiēsai krisin). “To do justice.”
To convict (elegxai). First aorist (effective) active infinitive like poiēsai before it.
Ungodly (asebeis). See Jud 1:4 and end of this verse.
Of ungodliness (asebeias). Old word as in Rom 1:18, plural in Jud 1:18 as in Rom 11:26.
Which (hōn). Genitive by attraction from ha (cognate accusative with ēsebēsan, old verb, to act impiously, here alone in N.T. save some MSS. in 2Pe 2:6) to agree with the antecedent ergōn (deeds).
Hard things (sklērōn). Harsh, rough things as in Joh 6:60.
Which (hōn). Genitive by attraction from ha (object of elalēsan, first aorist active indicative of laleō) to the case of the antecedent sklērōn. Four times in this verse as a sort of refrain asebeis (twice), asebeias, ēsebēsan.