You can attack Lian whenever you want. If she is alone, she won't harm you enough. Just make sure you attack her AFTER some trouble. Most of the time you can be sure she is completely on cooldown and thus an easy target. If she is NOT snipe her.
As Lian you always want to save your special abilities for Maeve. If you do not save it, Maeve will kill you in one single pounce, except you are double as good as her in personal skill. YOU are NOT a Maeve counter, so do not act like you're one.