On larger maps, invest all of your eco into improving your economy for a long time, by upgrading mass extractors.
If the AI doesnt play aggressively with low-tech units then they are forfeiting the map which means you get lots of extra mass points, and your economy will be stronger than theirs.
Once you have enough eco, its time to build something that can kill their base. No base can withstand the onslaught from tons of t3 mobile artillery. Obviously you will need some units to support them as well.
You should have eco enough to build your own experimentals. On large maps if you scout often enough you will have heeeeaps of notice to counter an experimental.
If the AI is building static t3 artillery (buildings) it is very very expensive and hard to do. If they manage it it's going to be hard for you to counter ... build lots of shields and then try and snipe their artillery somehow, probably with T3 Bombers. However, really they should never build this building because it's too expensive and if they are wasting their economy on that, you should be dominating them in some other way because you spent your economy better.
So: 1. get the map, 2. eco up, 3. build things that can kill their base