I played on the easiest setting in SP and I rolled over all of my AI opponents. However when a buddy and myself team up on line against the AI, they are far tougher and seem to have the ability to send an onslaught of units and have incredible defense in a fairly short time span.
There's no way it's set to the easiest setting on line, and I feel that the game is certainly not playing by it's own rules considering the amount of troops the AI can send our way. I wouldn't be surprised if the AI builds 40 percent faster than human players.
I don't care about the altitude, I just want another drink.
You can't get by for too long playing on the easiest settings on this game. What you are experiencing is really nothing compared to the higher difficulties. I would suggest you and your friend team up against the computer (if you haven't already) and just keep playing until you figure out your style. The computer can be tricky at times on multi player but you will soon realize the computer is not difficult enough. Just enjoy the fact that it is still that challenging. :)
P.S. make sure to make use of the feature that allows you to control what every population is doing in your city.