What ideas do you have? If your military is ahead of them in tradition and quality (or you can roll a good general), then you'll be able to defend yourself if you pick your battles carefully. The most important thing is defending forts that they're sieging on bad terrain, like jungle, hills, or mountains. When you attack stacks that are sieging these forts (that is, haven't completed the siege), they get the terrain penalty, which you can very much use to your advantage with strategic fort placement. Just make sure that you don't attack over rivers if you don't have to.
Your base ideas are slightly better, militarily, than Ming's are, and your tech is a major advantage - so if you've got Quality or Offensive, you should be able to win the majority of your battles with careful planning and good army management, despite their numbers. If you start getting trashed on manpower, start buying up mercenaries - it's better to spend the money now than regret your losses in provinces later.
