When fighting against her, the "Deep Roots" legendary is mandatory. She will reduce your healing anyway, and by rooting her you can render mobility potential unusable for a few seconds

Willo doesn't have the amount of air potential you think she does. Deep Roots will get her killed 80% of the time if she's in the open.

Willo's Deadzone completely negates Grover's ultimate, so a good Grover will avoid using it while you're nearby. Use this knowledge to help your teammates corner and kill him, but be wary of flankers because you're first on their hit list.