Or, perhaps, one should learn how to pray.
We took your argument seriously and respoonded to it. We addressed the argument you raised and pointed out that the text itself mentions "VAIN" repetitions and thinks that the repetitions will somehow cause our being heard. That was sort of the "negative" side of our response. Then I referred to Psalm 136 to show that repetitive prayer is found in the Bible.
All I look for in a conversation of this kind is that at least there be some acknowledging that the text you brought forward was able to do the task you gave it to do. There might be other ways to argue against the Rosary, but it seems that when somebody brings up the classic anti-Rosary text and it is addressed, possible even effectively, some kind of notice should be taken.
Speaking loosely: Say I have an idea and then go looking in the Bible for support for the idea. TO me that might be something like rationalization. It's not clearly the Bible that taught me, it's me looking in the Bible for confirmation of an idea I got elsewhere.
This impression is strengthened when I bring forth what I think is my "proof" from the Bible, and have it demolished, and respond by restating my notion in a way which suggest my hearers are lacking in some capacity.
So what do you think of the verse you cited in the light of Psalm 136?