Mammoth tanks by them self are useless, you need to back them up with some juggernauts, then they are nearly invincible.
The only way to defeat an army like that is either with another GDI army that's exactly the same  , or use loads of Nods EMP Buggys to knock the units out and finish it off with avatars.
I rather like the stealth tanks they fire fast and they're great for raiding.
Though Scrin is very powerful if you manage to get the time to create the best scrin air units. All you need is around 10 of them and they are unstoppable |