Brute strength does not determine the outcome of a fight.
We know Tsunade is stronger than Minato, but because of his skill, speed, and diversity, it is unlikely that Tsunade could beat Minato one-on-one.
A has enough endurance to tank several of her punches without dying. We know Orochimaru was able to tank Tsunade multiple times, but a argument could be made that Oro has freakishly high endurance.
A can't use any of his jutsu that requires hand seals since he only has one arm. That means he must rely on his lightning armor, speed, and his own brute force.
Judging by A's strength feats, his strength isn't too far off from Tsunade's. If it weren't for Madara's comment, many of us would have still believed A was stronger. A's punches can still cause incredible damage.
Tsunade hasn't displayed any high endurance feats, so I don't think she can tank A's punches or can move fast enough to catch/avoid the Raikage. She would need to rely on her healing abilities to last in the fight.
Although A's diversity in a fight has been drastically decreased with only one arm, his speed is a major factor that has only been rivaled by two ninjas (Naruto and Minato). If A can connect his punches to Tsunade before she releases her healing ability, and avoid Tsunade's punches (which is very possible considering his speed), then I believe A can win.