The GTI only attacks the generators by land only after they destroy everything else... in that mission i assembled about 40 Venons, and when the alien where nearly destroyed i sended all of then to cover the generators... you dont need much more than that... the rest is just a cleaning using nuclear strikes and the Vertigos bombers
The tanks attacks on your base are easy to defend, the GTI ALWAYS attack by the same side, just build 3 obelisk in the big tiberium hole and ready
About the scrins... well i just finished, it was short, only 5 or 6 missions, the scrins where suprised, a Tiberian explocion should be caused naturally, only when the planet was full of tiberian, they never expect that there will be forces on the planet, they should be already self destructed or extintion, because Tiberian grows to more than 97%, they come to harvest the Tiberium, (called Ichor, by them).
Well at the end "Mission Aborted, prepare a full invacion force, Earth will fall". and are looking information about Kane...