I have found that certain maps provide advantages over the computer that makes beating them much easier. I usually play a regular game at medium level with max pop 150 against 1 or 2 opponents.
In this article, I refer to a "defense post" as a gate with two laser turrets on either side, 2 AA turrets on each side, a fortress, and a shield generator. Build the items in the order they are listed as soon as you are able.
River - This map provides one bridge across the river per opponent (from my experience). Place a defense post at the front of each bridge, scatter some AA turrets in your main compound, and you're set up to win. Amazingly, I've never seen the computer use water or air transports to cross the river into my territory. Humans are not so stupid.
Forest - This map is similar to River in that there is only limited access to your base. Block each bottleneck with a defense post and you're set. The computer has never tried to blow away trees to circumvent my defenses. Humans are not so stupid.
Arena - I THINK that's what it's called when you have prebuilt walls surrounding your base camp. The plus is that you have prebuilt walls, thus saving you the cost of building them. The minus is that it will not keep the computer at bay for long. The computer will send it's forces to attack your walls wherever they are not guarded. The end result is that you must actually spend a lot more on defense to protect the walls on all sides open to computer attack. Luckily, I found that once the computer punches a whole in your wall, it will keep aiming for that hole. I recommend allowing the hole to exist instead of fixing it. Build up your defenses quickly around that hole (turrets, shields, units, etc) and rename your base Raid Roach Motel - they come in but they never leave.