Something you should always do is focus fire. If Ying has the Life Like Legendary Card, this is even more effective as the heal beam loses 30% of its healing ability, but if you spread out the damage, she will have 40% extra overall due to 2 beams.
Careful near edges as cassie can one shot you if she sends you out of the map. knowing this she may chase you there in order to get an easy kill. Try knocking her off instead or leading her to a team member who can if you need to retreat.
Cassie's Roll'n'Shoot combo givers her shot 195 bonus damage a shot if she can consistently hit you. and it reduces her roll's cooldown to 1s. If she rolls, try using a movement ability to make her miss out on the extra damage have to wait full time.
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.