There are few things you should not forget when playing this one - you should not allow anyone to take medium island, always have more armies (at least twice) than opponents so they won't attack. All of your empty lands adjacent to enemy must be reinforced with at least one army - that way you will not easily lose bonuses. And last, northern bot is your main enemy, so when you meet both, maintain strong presence in the south, not necessarily kill that commander, but do offensive over north one. Keep him busy trying to take back his bonuses and always do push one territory forward. When both start being about equal, you can finish them one by one.
I've been wanting to reply to this for a couple of days, but I needed to Level up to L4 first in order to post in the forums! It took me ages to crack this one.
Anyway, iirc then head north and south rapidly. Take the middle island fairly early and keep adding a couple of armies to the eastern tip on each turn to build up protection. Then sit while you take the south.
The strategy to the south is to track down the commander, who is always in the landlocked region Behemoth Valley about half-way round (at least he was for me). The quicker you get him, the quicker you can relax a little down there.
Attack the north aggressively and quickly. Take the Gnome Republic for the 9 armies per turn and head straight through the heart of the continent. This is where your main battles will be and you need to stop him gaining armies.
From what I remember of the history graph, turns 7-12 were crucial in not allowing the armies to the north getting away from you in terms of numbers, and from turn 17 onward, you should pull away from him. Then you can start attacking from the middle island and divide and conquer from there.