C&C yet again ...

... I just beat a medium AI in skirmish, without any handicap. The key seems to be getting rich quick.
The first thing to build is a refinery, the second is a mobile outpost to secure one of the free Tiberium fields.
Then you just need to get to the Mammoth stage quickly, and use those to defend your bases (by not using too many defence structures you also get them promoted).
From my experience training infantry to garrison the central buildings is a waste of resources. Once the AI comes up with flame dudes your infantry is pretty much toast anyway.
And not the good kind of toast either ...