Use your Mortar Teams to bombard Garithos' unit production buildings while your other units focus on destroying the sleeping enemies. When you get close enough to the town hall, Garithos will awaken. Try to make it so that the only unit you fight is Garithos. Put him to sleep if other units are attacking. Use life drain with Sylvanas to bring him down faster when you're ready to kill him. Eliminate everything here, except for houses and towers. Once that's done, take out the base to the north. During this battle you should begin making two crypt fiends and upgrading your ghouls to frenzy. Make about ten ghouls and four crypt fiends. Research upgrades at the graveyard when you can. When you have about six ghouls and two crypt fiends, you can start picking on units in Detheroc's base. Be sure not to let your ghouls wander until they have more soldiers in their army. Set the rally point of your two crypts to just outside Detheroc's northern base so that you can quickly add them to your attack group. Build spirit towers on the southeastern corner of your base to both raise your food cap and defend against Detheroc's inevitable attack.

When Garithos' base is devoid of any life, have your mortar teams keep pounding on it while Sylvanas and your other units go back north to pick on Garithos' other base. Again, try to destroy all units so you can leave mortar teams behind.

Chew into both bases, and when Sylvanas' work is done have her join your ghouls and crypt fiends, who you should have been reinforcing. It's entirely possible to eradicate Detheroc's northern base before the time runs out. This will leave you with a single base and you having a full army to deal with them. Wait on their attack before you make your push though, as you can fight them on your turf. You can block some of the incoming attackers by having Sylvanas and her army stand at the northern exit of their first base. In my case, only Detheroc made it to my base and four spirit towers took him out. At this point, both of your heroes should be nearing level six. Take Charm for Sylvanas and Doom for Varimathras. When you finally decide to assault Detheroc's base, use charm on the frost wyrm when it attacks and pick a weak unit for Doom so that the doomguard will arrive quickly. Take out all his production buildings to win the chapter.