She is more experienced she is one of the best medical Ninjas which means she knows the anatomy second to none. He was so skilled she was able to hold charkra supply for the occasion that she need a great deal of healing.
Tsunade- http://youtu.be/tnLPg74S5dA
Danny has to use the Iron Fist to generate that much force and damage. Tsunade using her super strength punches is her standard method of attack. So the only way Iron Fist hits harder is if he uses the Iron Fist. Even then, hitting her would be the problem, and taking her hits would be an even bigger problem.