I don't know if this is still possible in SC2 because I'm still working through the campaign*, but in SC1 there were a lot of micro maps where it was possible to beat the hardest AI 7v1 with any of the races (although Zerg was VERY tough because they couldn't turtle before the first rushes due to having to wait for creep to spread.
The strategy was to just turtle, then either build an overwhelming air force, or in the case of Terrans build siege tanks to wipe out your immediate neighbors and then just expand from there. Once you had a second base you could build out unit producing structures and eventually wipe out the AIs one at a time.
It was often hit or miss though - sometimes as Protoss a Terran AI would get siege tanks and it would be impossible to hold them off long enough to get a counter. As Terrans you might get hit by stealth units that could hit your blockading supply depots from out of range of your turrets, or you might face siege tanks or mutalisks that could overwhelm you.
*hey, I work and have a family, what can I do :)