According to Newton's laws of physics, everytime Tsunade punches or kicks something, her body takes the same amount of force in return. So if she has more power than A that means her body can take more damage than he can deliver as a result. Furthermore, Bee's Lariat is more powerful than A's and it only cut Sasuke's neck. There's no way A's lariat would decapitate Tsunade. We also know she can heat from a slit throat as she healed from it when the heavenly transfer technique was used and that healing didn't require the bulk of her chakra. Besides that A's entire arsenal is limited to one basic technique. Combine this fact that he has low intelligence and I see Tsunade who has a 5 in intellect coming up with a strategy. Besides Tsunade has more in her arsenal. She has Katsuya, she has her chakra scalpels (judging by the way she said Kabuto's precision and speed may surpass her own in her prime, we know she's capable as the same attacking style he is), granted body pathway derangement may be moot on him once he's in his cloak. People say A has bijuu level chakra but there's no reason to really doubt Tsunade has less, she's said healing requires a lot of chakra yet she had enough chakra to heal konoha. We've seen her heal a slit throat, and very other cuts on her body, in addition to regrowing all the organs she lost when she was stabbed , and then regrowing her stomach again after being caught with 3 yasaka magatama, in addition to healing the kages during this fight.