Shadow Madara was able to take out Godzilla sized beasts with nothing but punches.
How?
We all know that anti matter and matter collisions release tremendous amounts of energy( much more than a nuke).
Well, shadow objects in Limbo realm could be considered Anti Real.
When Anti Real and Real collide, a huge impact should occur.
Here is the caveat, when anti matter and matter collide, they cancel each other out, just like when -8 encounters 8 to become 0. In anti matter though, the masses cancel and are both converted to pure energy.
But Shadow objects in Limbo do not follow causality. They do not follow the rules of normal reality, hence they are like Looney Toon characters and can never be destroyed. Think about it, Limbo is nothingness personified so how can something that is "nothing" follow logic?
The only way to force Shadow Madara to follow logic and causality, hence allow it to be destroyed, is to hit it with Rikudo Senjutsu.
It seems Madara is able to decide the precise moment when the Shadow Madara is able to interact with the real world.
This is why Madara was scared of using Shadow Madara to retreive his Rinnegan. Even if Shadow Madara tried to gently remove the Rinnegan from Obito, just coming into contact with him would cause an extremely violent reaction.
Another possibility is that by breaching the Limbo world and real world. (i.e Shadow Madara's Punch), a huge surge of energy is released from that small instant where the walls of reality melt.
