Taking China with America is relatively easy. It can be done in 2 wars, though if your game is anything like mine Russia will pounce on China immediately after the first war. I'd suggest taking the Damming Region in the first wave as it's a money factory and a huge draw for Russia. Fighting Russia would probably be more difficult, especially without a European ally. Part of the problem is that Russia doesn't have much to take and only loses it's military effectiveness after several wars. After occupying Siberia for the 4th time leaves me pondering the whole idea.
Truthfully the USA is one of the best countries no matter the objective. If you want a challenge, pick someone else. I've even found ways to make Prussia interesting for the entire length of the game.
