Most of Willo's kit relies on area denial. After Seedling or Dead Zone have been used, it's better to stay out of reach. Remember that she has quite a large cooldown for those abilities!
As a support, be wary of Dead Zone! Do not stand inside it unless it's unavoidable and do not attempt to use healing abilities on allies affected by it until they've been cleansed. Remember that the effect lasts for 2 seconds after exiting.
Deadzone is not completely ground-based like Damba's gourd or Tyra's fire. It works like Viktor's ult which means that it can be blocked with shields as long as you're not exposed to the middle.
Do not rush the objective when there is a deadzone placed. This means playing passively and prioritising targets on the enemy team while waiting for the deadzone to disappear. Once it's gone, the team can group up and start to slowly cap the point.
Willo's Dead Zone ability is very effective against 'local healing' this includes grovers healing, Grohk's totem and similar things. Dead zone will prevent deployables from being effective and will force champions like Grover to move out of the zone.