PvP is a good way to make money and grind Xp. THis guide should help you become a force to be reckoned with.
First- Pay attention to levels. Five levels higher, that's about your limit. Ten or twenty, and there is a high likelihood of severe casualties and retaliation.
Use your spies. When it loads, open the battle menu. It will list the enemy's units and defenses without you having to look. Past that, you must look around to find the exact nature. Stay near the water, and just hug the edges of the city.
Find a good place to start. For level 25 and above, that is the top left corner of the map. The bottom is good. Pick the corners to start in.
Don't waste time on the army. Go straight for the command center. It's faster, and less costly that way. Only demolish enemies you simply can't afford to have retaliate. Mortar Throwers and Demolition Tanks are good for clearing out turrets, so use them if available. For people with mechs/dragons- use your drones/draggies. They don't cost anything, and can be used infinitely. Also, never abandon your mechs/dragons. As strong as they are, they will be destroyed by swarms. Their greatest enemies are numbers. I've seen twenty Soldier IIs take down a Agamemnon Fortress. It can happen. Fight smarter, not harder. And remember, anything you do, someone will likely try on you. Build your city in a way that you couldn't beat it if you fought against it.