Bigger spoiler here. This has some solid advice for beating challenge 2. I think a combination of strategy and luck are required, but you don't need to play the perfect game in order to win. However, if your game isn't near perfect for the first couple of rounds, just start over.
The Zerg-
Here's the list of bonuses I gathered during the zerg phase, by turn:
2-Central America, India
3-West Africa
4-Middle East
5-West US
6-Caucasus
7-East Africa, East China
8-East US, South Africa
Followed by a quick succession of Canada, W Russia, Indonesia, N Africa, and Central Russia. I didn't secure Greenland, Europe, South America, or Scandanavia until later in the game, after contact was made.
The armies per turn went roughly:
5
5
12
16
20
25
31
39
51
56
66
71
77
That list isn't exact, as I was scribbling notes from several games and got them mixed up. The point is: every turn you add a bonus, and every turn you set yourself up for the next bonus.
Contact-
Follow my route, and you'll contact the AI in Svalbard, Australia, Anadyr, Canada (get to Nunavut first), South America, and Europe/Scandanavia. If you're lucky the same AI will be in South America and Australia. If you're really lucky (I was) you'll also get the same AI in Scandanavia and Canada. This allowed me to block AIs #3 and #4 with Svalbard, and the AI #5 at Anadyr.
Plugs-
It should be obvious already that you have no hope of fighting all five at the same time, and that all five will find you if you don't drop a plug. I dropped my first plug on Svalbard, which I highly suggest, cause you'll probably tie up two of the AIs with one plug. You drop your second plug on Anadyr, and you need to get to it before the AI does. You'll spend 3-4 turns defending Anadyr before you get the second card, and it's critical that you hold it.
Cleanup-
I doubt you can make it to all the choke points with enough men to hold them, so you'll need to do cleanup. Indonesia/Australia and South America are easy areas to hold and clean. It took me four or five turns to secure Europe/Greenland/Scandanavia, but it can be done.
Honestly, if the AI gets loose in Asia or Canada, consider starting over.
Mid game-
I broke through the AI's defense in South America and attacked Africa, and then moved on to everything else. Australia/Indonesia was mostly a holding area for me, as was my South America.
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That reverse abandon must have been nerve wracking.
