Hi. This was the first level my ass was handed to me also. On previous maps, you could dig in and build up base first, explore later when you have strong parties. In this mission, you don't have the luxury of time. YOu need to get exploring units out ASAP, uncovering the map
key things which may help you in this mission
- each guild house you destroy nets you 5000 gold.
- in subquest you can make 15000 gold escorting a caravan.
- each trading post you uncover in exploration becomes your own
- destroying all guild houses gives you a nice surprise bonus
- don't upgrade your palace, I strongly doubt it could pay for itself in this level
- unless going pure rogues, I wouldn't upgrade any guild past level 2.
- avoid holy ground with fear flags if necessary
I used "blitzkrieg" tactics - just 6 rogues (upgraded one guild to level 3). Also succeeded with 3 rogues and 3 clerics by using protect flags on rogues to get clerics in on the action. That was slightly more challenging but even more fun.
I charged 6 brand new rogues straight at the rogue house near starting point. Earned enough to upgrade rogue guild to level 3, blacksmith, statue, marketplace items and a few upgraded guard towers to hold the pesky minotaurs at bay while you are exploring the map. Apart from flags, that is all the money I spent! Explore all the way to the ends of the map in a couple of different directions at a time - but destroy things one at a time with a group. Sometimes I didn't even destroy all the rogues guilds but I would always uncover 2-3 trading posts and do the 15,000 gold escort mission.
Edit: Only just thought of this, but on ANY map, the magic bazaar should ideally be the first building built after the wizard's guild, since earning 200 gold per day, after just 13 days, it has paid for both itself AND the wizard's guild! Even if you do NOT intend to get wizards out on this map, it is worth building these two buildings as early on as possible for the bazaar's income. Plus you can research enchantment in your wizards guild which also pays for itself after just 3 heroes use it

I never research the potions though.
The prima game guide shows this mission can be done with all 5 guilds provided you don't upgrade any guild beyond level 2. Otherwise, strategy is much the same. The time you lose at the start with more expensive guilds churning out more heroes will hopefully pay off later on when you have lots more heroes to multitask.
If you act swiftly, you'll make the gold and complete the mission before the elementals become a problem. (I think a single elemental had just arrived and was flinging one of my rogues around when I last completed the mission!)