Furies are weak and only good against slow units. There's no need to even talk about regular harpies. Orcs have much scarier units like Crushers, and especially Demolishers. This unit, in skilled hands, can break through almost any unit, even elite ones. The only difficulties might arise with archers and slightly with flying units. All ground units are easily controlled with goblin traps, which prevents neutral creatures from focusing on anyone. Demolishers simply take out enemy stacks one by one.
At the start, you get one stack of Crushers (which are upgraded as quickly as possible) and the rest are single goblins + the main goblin stack. Then you add Snatchers, which increase the mobility of the attack stack, or reduce the mobility of neutral creatures, allowing you to control them even more effectively.
Before I read this message, I was playing as the Barbarians with a mage, because I had a Dynasty weapon - the Staff of Eight Ipostasies of Ash, level 2 (+3 magic power and +30 max mana). This meant that, firstly, I started with harpies, and secondly, I had a lot of mana. So I simply took the Restoration specialization for my hero, upgraded the harpies in the first week, and took out some of the best troops.
But that's with a Dynasty weapon. If you don't consider the weapon, then choosing a mage for the Orcs is still a bit questionable. The Barbarians have too powerful units.
I thought about it and started playing as a warrior. Demolishers under the command of such a hero are truly formidable - they can take a hit and deal massive damage.
I just don't understand one thing - why divide the goblins into single units and a main stack?