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.
Ying is limited to 2 stationary Illusions. If Ying wants to push up she needs to sacrifice illusions to make new ones to heal or her team will need to fall back for healing. Killing her or her Illusions gives no healing for the enemies to retreat to.
Ying's Ultimate ability, Illusory Rift, heals but can let her use her Dimensional Link to teleport to her team. If Ying uses this right after respawning, look out for her teleporting to her team and kill her to waste the Ultimate.