Keep your distance, keep cloaking, and don't stay still. The amount of time (rediculosly long, IRL it would be much shorter) for the mammoths shell to cross a distance to hit you, you can move to avoid it. It enrages them beyond belief to be unable to even hit you. Then stealth up, lob some missles at them and run away. They always back down, frustrated, unable to hit the stealth.

Up close and personal? Let's not talk about that . That takes a bit of fancy driving (which the stealth can do, it's so quick and sexy ;D)

As far as GDI GT vs NOD OB+Turrets, I'd say they both have their advantages. I find the GDI tower much more menacing, and virtually unviewable/approachable as infantry. The NOD tower on the other hand can only shoot at one target at a time, is much slower, and the turrets are usually "dumb". I can get into and destroy the obelisk (sp) much easier.

Either way, rushing into the base with an APC to hug either tower renders them defenseless (don't shoot close range, either of them), allows you to get into the tower to kill it (c4 on the term). APC on both sides has plenty of HP to make it in despite a heavy barraging from the towers and arbitrary units running around.