The Mammoth is just a giant, heavily gunned and armored cardboard box. If you're smart enough, almost anything can destroy it, since you can run/ride circles around it and the turrent couldn't catch up with you. It should never be ridden around alone (of course, since it's been said many times) and only should be ridden at all if you think it can reach the enemy base in one piece, which is it's only use, really.Besides, if you're looking for field superiority, would you rather run out with a giant box that moves slower than a 4 year old on a tricycle that you could shoot with a pistol from across the continent, or a small cardboard that moves fast and is hard to hit?
"Take care of your cardboard box, and it will take care of you." -Solid Snake