The opposite of ‘defenseless’

It might be defended, or defensive.

If you're asking for feedback or coaching or an education, neither is going to help you very much.

The person who has ideas that are well defended isn't going to be able to listen carefully for the lessons that can help him change those ideas.

And the person who is defensive not only won't hear the ideas, but he'll push away anyone generous enough to share them.

Defenseless is the best choice for those seeking to grow.