1. So what if Tsunade has super strength; do you really think Minato will let her touch him? Do you think she can even touch Minato?
2. He was already the 4th Hokage. Have you ever heard of something called retire? The third was already too old, so he stepped down.
3. Then it's not a big deal if you have super strength. Also, it is extremely funny how you compare Tsunade to superman. Can Tsunade fly or have laser vision?
4. Minato was jonin and still became Hokage before the legendary senin. That proves how strong Minato is.
5. Kakashi > Shizune and Sakura.
6. Do you think Tsunade can heal when she is fighting Minato? Minato would be probably behind her holding a Kunine ready to slit her throat.
7. Minato has more Jutsu than Rasengan. All I see Tsunade do is jump up in the air and smash the ground. It can be easily dodged.
Ps. Don't forget Minato can still learn Sage mode if he was still alive. Also, Tsunade had the most stupid weakness I have ever seen: Blood. She can't do shit when she see's blood. She got pwned by Kabuto, too.