2 or more Mammoths laying seige to a base cannot be beaten if they have support and don't stop firing

if you are a good player then you DONT let infantry get too close, and you use your missiles to kill them

I can easily destroy anything that attacks me if I'm in a mammoth tank and I don't stop moving forwards.

The easiest way to destroy a Nod base is to simply keep moving. As long as you don't stp for repairs, but keep on pounding theier base, and ignore the tanks and units then you won't lose.
Because the great thing about the mammoth is that you can ignore all the tanks and infantry that are shooting at you, because you know you will do enough damage.