Willo is very effective with Maeve when she uses her granade, and she kill Maeve easy. Try to use your escape to avoid immediatly that granade and escape , or, use the dash to escape and use nine lives to heal. Then go and kill her.
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
You can re-position yourself out of the AoE of Willo's Seedlings using Rocket Boots, though you may end up sacrificing some Turrets and Barricade in the process.