Small start area with all mines but sulphur one.
I had to leave gold mine as it was guarded by horde of succubus mistresses.
Others were demons, cerberus. At start go with Findan claiming those mines, and buy a boat and head with scout on sea.
I didn't go underground as pit lords guarding entrance would be a bit tough.
Anyway, explore a bit sea with scout, build town as much as you can (I had to trade a lot) and return with scout home.
Now I've used a little trick. Both heroes knew Summon Boat spell, and we had a boat there.
Seems that AI knew the spell as well...as saw one succubus hero with a bit big army summoning my boat and heading to sea.
First I was pissed, but as that hero stopped on one island to kill some druids, I summoned the boat back.
Hopped with Findan and all the army I could raise and head to their land.
And immidiatelly after summoned boat with scout, and hopped in it. Succubus hero got stuck on that island.
In their land I took fully built, and poorly defended inferno town.
Silo there produces sulphur, so finally could've built treants.
Anyway, turn after got attacked by another hero with some big army, and I would bet anything that it's scripted event, as haven't seen the hero before, and he was in part of map with...well, nothing there.
But, with help of castle and mighty hunters, stayed alive. And kicking!
Claimed all the mines there as well, and after some time it seems AI bought some boats of their own, as succubus hero summoned one, and headed for my sylvan town.
Panicked at first, but as it turns out...hero was pretty bad, my scout succesfully killed the enemy.
And that after it took that gold mine (and seemingly lost nothing vs horde of succubus mistresses? I smell foul play!).
Anyway, there might be some other heroes coming close via water, but easily disengaged.
Two more inferno castles are there, both long way via sea.
With help of devils and pit lords, plus with regular sylvan army (beware of morale penalties - but it seems it affected only my inferno troops), wasn't that tough at the end.
Oh yes, main objective is to gather 20 emerald dragons, two ways to do it.
First, you'll be able to build dwelling in town after some event (think when you take first inferno town), or you can visit the obelisks, and dragons will appear in some places.
And always join you.