Ok, so after quite a few tries, I have found the best way (and have won doing it) is to ignore the boss unit until you have over 100 income. Always clean up behind him though so he only ever has one or two territories due to the boss unit.
This means the number of armies the boss has doesn't actually matter.
Never use a blockade card! There's no point.
Expand as much as you can and treat the rest of AI 1's land as a normal enemy, it should be easy to kill him. It is possible to expand in every direction for a long time.
Once the boss starts eating his way through your land, just begin the clean up operation and keep armies chasing him until you have hundreds spare.
Regards going to europe or asia, you should probably focus more on asia, but you may end up spending more on asia anyway as you needed them to defeat AI 1's territory there. It is important to defend greenland, and attack through france as it may take a while if you don't start early on. Britain is usually well defended so don't attack there for a while.

You'll know when you've won.

ZxCtyCxZ participated in compiling this information.