To answer an earlier question, I've completely eliminated the USA on admiral difficulty playing as the Japanese. Here's how I did it:
Step 1: Japan should always launch an early war against Russia imho (not China - they can be dealt with later... bwahahaha). With good micro-management and by making use of your fleet you can take out Vladivostok and/or Port Arthur early. This gives you a nice industrial base to expand into Siberia later. Seoul and Vladivostok are both great cities if you develop them properly.
Step 2: Take Seward, Midway, and Hawaii. This is very easy. Seward is a nice colonial city once you mine out the surrounding mountains and is a good place to build the Great Gold Rush.
Step 3: Most Japanese players will now go full-throttle against China, the Philippines, or expand further into Siberia. All of these are sound strategies. But, since we're taking on the USA, you are going to want to send your fleets across the Pacific. By this time I had two corps from my war with the Russians which really helps. I'd also built a massive fleet. Your destroyers and BBs are great for redlining the defenders of coastal cities. I waited until the US went to war with Mexico... Gave them a few turns to throw their troops south... and then hit coastal California. The trick here is TIMING. If you wait until Uncle Sam has cleaned out Canada and Mexico then you basically have very little chance of invading North America successfully (and much easier targets elsewhwere). What you have to do is hit the US as it goes to war with Britain and Mexico. Their weak point is either San Diego or Seattle since you will then have friendly Mexicans or Canucks on one flank instead of being completely surrounded.
Step 4: Once you have a beach head established, start shipping over massive amounts of infantry and howitzers. Use your artillery and your corps to cut US rail lines leading to California and Mexico... This makes it much harder for them to counter-attack.
Step 5: From here on in, it's basically a massive artillery grind. Once the Rockies are secure and you have a good number of corps up and running you should be able to gradually grind across the continent. You'll need to keep sending a steady supply of men over from Japan, but, as an island, you have the luxury that you can afford to leave the Motherland lightly defended. (Note: I would NOT recommend this strategy with a continental European power attacking from the Atlantic because you'd have to send so many boys across the Atlantic that your greedy neighbours could hit you back home in Europa! Japan and the UK can do it safely as islands though.) The one thing to be careful of is that Russia doesn't reverse your gains in Siberia/Manchuria/Korea. But the Russians usually have at least someone atttacking them from the west so are unlikely to have the resources for a major hit on you in the Far East. Once you get Industrialization 2 it's problem solved. Just pay Germany or Turkey or someone to invade Russia from the west if Ivan starts causing you problems.
And that's how I conquered the US with Japan on admiral. Just remember: TIMING. Don't wait for them to conquer the Canadians and Mexicans and THEN decide to attack them after they've held North America for a few uneventful years and built a ridiculous horde of troops. Hit them hard and early while Mexicans and Canadians are still around to divide their attentions.
Also, I am opposed to reducing the power of the USA in this scenario. Even by this period, the US was an absolute beast once it got rolling. I think the scenario is done well. France, UK, and Germany have better industrial infrastructure than USA to start (accurate), but USA has greater potential once it develops its huge lands (also accurate). Conquering the USA (and to some extent Russia) should be the toughest task your army can face in the scenario because that's accurate, just like conquering Britain should be the toughest task your navy can face. It's realistic imho. I like keeping the US strong.