Batch set with merge option
If you're using Python just use:
batch.set("path/to/document", data, merge=True)
Firebase documentation - addition of data says that we can add a parameter "merge=True" for setting with merge. The same applies to batch updates and works fine when tested on Python3.
After some time, I found out that you can achive that using:
batch.set(db.doc(t.ref.toString()), { field: admin.firestore.FieldValue.delete() }, { merge: true }))