Use your grenade to hit behind her shield; if she turns around to block it you can get free shots in. Don't fire on the shield unless you have high Wrecker, because she can hit you during that time. Once the shield is down, fire away.

Lian's biggest advantage is superior range/pokes. Use the map to your advantage and try to sneak up on her, and once you're close enough you should be able to take her out. Use Shadow Step to evade her Presence and especially her ult.