I have never lost a mammoth to any unit one on one. It's when you are dealing with a bunch of Nod forces and the stealth tank sneaks up behind you... that's when it's easy to take out an unsupported mammoth. Direct hits to stealth tanks by a mammoth are deadly. I think one mistake people make is to aim with the tank in the center of the circle. For most units this is fine, but the mammoth's two turrets aim slightly differently (the two dots on the left and right side of the targetting circle), so to be exact you have to rotate the turrets slightly left and right as you fire in order to get direct hits.

Properly supported mammoths are deadly. Raveshaw is good against them.. until you're picked off by a sniper. If defending your base, several of those rapid fire black hand units will eat apart tanks pretty well.

As far as the AGT is concerned.. flame rushes usually take that out pretty quickly, unless GDI has a ton of tanks still at the base.

I've played enough games to know that anything's possible, but a GDI team full of good players and teamwork is really tough to beat unless you cripple them early on. Except for maps without defenses... then Nod's stealth ability is extremely deadly once they can afford nukes.