Try to predict Lex's auto aim ability and deflect it! This should be able to take Lex down to a low amount of health. If you aren't able to counter ti it will all come down to who has better aim.
If RMB, you Reversal. Reversal will reduce Lex to low HP. You should always pick the Dark Stalker card. A good way to counter him Is to just out-damage him. You survive more shots than he.
Androxus can deflect Lex's In Pursuit ability, also Androxus shoots faster making him more forgiving for missed shots. Although Lex deals more burst damage. This match up is based on whomever goes first.