Arthas will use brute force tactics. Available to him are abominations and ghouls. I don't recommend you use too many abominations, but you might want four to accompany Arthas initially. Keep four up at all times, the rest will stay ghouls. Leave about six ghouls at the base to watch for refugees. Slyvanas' base is a bit different, having banshees instead of abominations. Obviously, your goal with them is to posess enemy units. I recommend you save them for knights, especially those that teleport to villages with paladins. A paladin comes with 2 knights, has both of his friends posessed and finds himself surrounded by undead. This is ideal. Kel'thuzad's base has necromancers, and they are probably the easiest to assault villages with. They can raise armies very quickly as long as a few units have died.

Set each base's production queues to make their types of units so that some are left in the middle of the road. The first base I recommend you assault with is Kel'thuzad's. Take Kel'thuzad, along with five ghouls and six necromancers to the northwest. Find the first village you can and assault it. Make sure the necromancers raise dead automatically and use frost nova often. When the paladin comes, focus on the knights first because the paladin will use divine shield. When it wears off, hit him so he can no longer heal the knights. Finish them all and the first village should be down. Head Kel along with everyone else back to his base.

Now you can use Sylvanas' base. Take Sylvanas and about four banshees, with seven ghouls to the west. There is a sorceress hero there, who has slippers of agility +3 for Sylvanas. To get them, you'll need to run past the cannons that fire from the Human base. Return to your own base and replace any units you lost with Sylvanas. Now hit the Human base that was firing at you as you went after the slippers. Sylvanas can hit a total of three bases this way. You should possess anything that looks powerful along the way. Since gold is no concern, you can keep the banshees coming. The other two heroes should guard their bases at this time.

Kel can hit a couple bases himself, as can Arthas. Continue this pattern of having the other two heroes guard their roads while a small force takes care of the hero who's away. Arthas should use death coil to finish off knights, and than heal his own units. This will end the battles sooner. Work everyone toward the center of the map so that the only thing that is left is the human town guarding the altars. Surround the city with undead, and go in with everyone at once. Use their specials -- possess knights, death coil liberally and use necromancers and black arrow to raise skeletons. Death and decay from Kel'thuzad is great on buildings to soften them up.