The pain you can feel is multifactorial:
It depends of you, your own sensibility, and most off all of the injector.
We can first take time to relax you, to explain you the treatment, take a very thin needle: 33 or 34 gage, and inject slowly. You will always feel the difusion of the botox in your muscle, and I think that a topical anesthesia will never be enough to avoid this feeling.
But don't be afraid, nothing strange, if your MD takes care of you, you will only feel a discomfort.