You can do some creeping right away here. There are many dire wolves and giant spiders for you to slay and I got enough XP here to gain Arthas his seventh level. Use holy light to heal your units and if you lose any, use resurrection! Among the treasures I found in the caverns were a greater healing potion (not so useful) a scroll of the beast (saving that for later) and an orb of frost (really useful). The orb of frost adds to Arthas' damage and slows the attacks and movement of enemies. It's one cool item.

The gold mine is pretty far away from your start location (all the way to the east) so do take some time revealing the map and getting that experience. We'll be able to move faster once the base is started that way anyhow. Once you're done, head to where the gold mine is and you'll encounter Muradin, a new hero unit! I love this guy too. The slowing attack is invaluable. Use it when the dwarf is surrounded to weaken the enemies.

I was attacked pretty early here by a group of gargoyles, which are a total pain in the ass. You only have one gyrocopter here, and the swarm of gargoyles destroyed it very quickly. I used the three riflemen you start with and had Arthas heal them. They destroyed the gargoyles and I now had a new priority: air defense! All I wanted was to create a medium group of 5 or so riflemen and keep them hotkeyed, guarding my base. I'd recommend you do the same. Don't negelct the ground forces here either but remember you've got some attacking to do.

Cannon towers are available now to guard your base and one to the north/one to the northwest couldn't hurt as that's where most of the enemies are coming from. Northwest of your base there's an Ancient Windigo creep you can kill which will drop gauntlets of ogre strength. I'd give it to Muradin, personally, and you could even give him one of the orbs Arthas has.

I recommend upgrading your melee units' armor (primary) and attack (secondary) at the blacksmith before you go attacking. You don't have to do it all the way, just get the first level or so and then research as you go about your business. When you're ready you can do the optional quest here:

Optional Quest: Rescue Muradin's Men
Pretty obvious where to go as the base has been revealed to you. Rescuing the men is as simple as destroying the undead base that is right next to it. I managed to destroy this base fairly early on. As a reward you get a free outpost and the ability to build steam tanks which are very effective at destroying buildings. Unfortunately once you've freed the men your population cap may reach the maximum. In this case, don't leave your base undefended but try to do some damage to Mal'Ganis' base. Which is located...

West of the dwarven base you'll find Mal'Ganis' main base. I managed to destroy it without too much trouble with an attack group consisting of Arthas, Muradin, two knights, three riflemen, two footmen, a steam tank, a mortar team and a priest. If you fail with such an attack force just be sure to get the heroes out of there alive and you can easily replace the units you lost and get right back in the fight. With two bases you should be able to do this fairly quickly. I didn't face anything too formidable here as well, Mal'Ganis had been sending out many small attack forces, the thunder clap skill is so useful and Arthas and the priest had almost no trouble keeping my units healed... to a point. Eventually my units started dying off when Arthas ran out of mana but the damage was done. Mal'Ganis had no hope of recovering from my initial attack and the remainder of my forces wiped up the base.