You begin with Arthas, Muradin, two knights and three riflemen. You will have to worry about the base to the east of you as it will be under attack pretty frequently. Make decent defenses at the three locations you need to defend. Don't worry about exploring the map on that side. Worry about Frostmourne and keeping what you already have. Build up an economy (gold and wood gathering) and make a few knights. You can build Gryphon riders now for the first time. They're pretty powerful and can attack ground units from the air. Thus, good base defenders. They can learn the storm hammers ability which lets them do some extra damage.

Advance with Arthas' party and stop when your base comes under attack so you don't have to deal with two battles at once. I had an unfortunate incident with some spirit which posessed my units and caused them to destroy each other (I'd never seen that before) but, even though they were knights they weren't very important as Arthas and Muradin are so powerful now. I could have resurrected them with Arthas but I wasn't worried about it. Muradin has the avatar ability if things get dangerous and Arthas has divine shield. Not too far past this you encounter the guardian and his party, if not for the nasty incident with the posession, would be the most dangerous to me. But, remember you can use Muradin's skill to stun him. With that, Arthas claims Frostmourne.

Now you'll have a new objective. Finish off Mal'Ganis. Arthas is freaky powerful now and with the treasures of Muradin at his disposal should not have any trouble surviving on this level. I believe it may be possible for him to do it all himself, but you shouldn't be careless. Take at least two knights, two priests, two footmen and a steam tank or two to assault Mal'Ganis' forces. Use some gryphon riders, if you like, as some air support.

Begin by assaulting the base to the east. It's very close and they are sending forces to attack your base. There was nothing too powerful here and I did manage to destroy it with a relatively small group. Only Arthas and the knights survived but I was left with some satisfaction knowing I had curbed some of the attacks. With the first undead base destroyed I replenished my forces and prepared for the next assault - which will be Mal'Ganis' main base.

The main base is protected by plenty of air units including frost drakes. So bring a number of anti-air capable units. I'd recommend some gryphon riders or plenty of riflemen. You will want to bring some steam tanks as well and may even want to use multiple attack groups for this one. If you're attacking Mal'Ganis' base it stands to reason he won't be attacking yours so try to divert some of your defending forces for this, the final assault. Use every resource available to you here. There's no secret to defeating this base you just have to bring a diverse attack group. Even if you fail, try to do as much damage as possible and get Arthas back to the base to replenish your forces. At least you destroyed like 10 of Mal'Ganis' buildings, right?

You may even consider spending a couple minutes at low or no upkeep to get the gold gathered for another try. High upkeep can be a real pain in the ass. Upgrade your forces durnig this time instead of building them. Make them more effective than the last time you tried. When you finish off Mal'Ganis' base you finish the Human campaign. Well done, now watch the ending and prepare for the Undead campaign.