This was the approach I took to clearing the challenge. It was a bit tricky as one AI definitely dominates for a while
A very helpful strategy I used with neutral territory capture with 3+ armies was to weaken it using 1 attacks when possible because it reduces the total armies needed to capture a territory from 5 down to 4 if the neutral has 3 armies and 8 to 7 if it has 5 armies. (There is no difference if it has 4). You need to own 2 adjacent territories to capture the territory on the same turn or wait an extra turn. If the territory didn't help me complete the bonus that turn unless I needed it to have all territories visible of the bonus, I would weaken it until it had 2 armies.
First 3 turns: Complete Nest Woods Coast, use 1 army to weaken DZ6 and DZ3 down to 2
Turns 4-10: Complete Burnwoods coast while weakening a territory from Nest Eggs Cliff, when its down to 2 or 3, capture it on your last order to prevent AI 8 from being a threat to you. Also expand Southeast to take a Giant Yards territory.
Turn 8 onwards: Your primary targets are AI 8 and AI10, if you get them early enough, you can take most of their territories without needing too many armies because they will be hitting neutrals. Once you reduced them to be non-threatening, your new targets are AI 1 and 9.
Once you secure most of the Giants East and Beast Legion, focus on breaking the bonuses of your biggest threat AI 11, AI 5 or AI 6, use reconnaissance cards to get an idea of who the alpha is. and break their megabonus. Don't worry about sea territories lost, except to protect the one near your bonuses. While you are at it, take he SKull Warriors and if feasible, the Eagle Warriors region. At this point, you'll be strong enough that its just cleaning up the remaining forces of your opponents
The reason for the order I'm suggesting is 2 benefits
1. You won't face more than 2-3 opponents in the beginning and no more than 5 when you become established.
2. Almost every opponent if you go in this order already has another opponent making it easier for you to take them down
How to skip: See this thread, just note it only works for the latest single player level you are up to https://www.warzone.com/Forum/379567-spoiler-skip-level-single-player-mode
Edited 8/17/2019 12:22:42