I believe the shortest amount of turns on insane are 26. I wouldn't believe any of the records on Kongregate because there is a hack out there where you can have unlimited troops.
As for strategy, there are a lot of threads out there already explaining basic strategies so scroll through the forums and see what you can find. Some quick notes:
be always expanding
know how the AI acts in new territory (tends to favor the neutrals)
find choke points and abandon them at the right time when the AI is going to attack with a big stack
bonus break them as best you can
if the AI gets into your main territories contain them well.
There's a lot more, but those are some basics to help out. I don't have the time to look but someone could post the forum threads.
Also, if you feel necessary, use this youtube clip as a guide.