So you are enemies with the Undead? You should enjoy this page.

Rush!
Undead players are very weak early on if they don't have a Nerubian tower. Many Undead players like to upgrade very early. They assume no one will attack them because of their towers or tough defenses. That is their own undoing. It is especially useful in team games to rush one of the Undead players. Your goal will be to kill their Acolytes. They may try to run them away (if they are decent) but you can still kill them pretty easily with certain Heroes and skills.

[ Click to Enlarge - 154 KB ] This town has no defenses. The players see their opening.

Kill Their Haunted Gold Mine
If you can get in early at the start of the game, or do some hit-and-run attack, you can severely damage the Undead player. They will have to resummon their Haunted Gold Mine which will take a while. The counter to this is repair the Haunted Gold Mine and to have some towers around it.

[ Click to Enlarge - 155 KB ] An Undead player, Crimson, attacks another Undead player's Haunted Gold Mine. He puts so many Ghouls and units around it that the enemy player cannot repair the Haunted Gold Mine.

Hide Fun!
Night Elf players (or someone with a Cloak) can use Hide to harass Undead Acolytes on the Haunted Gold Mine. Simply attack, then hide. Wait a while, then do it again! This makes Undead players very angry TT! Undead players can counter this by using Dust at their shop (oops shouldn't have told) or by building a Nerubian Tower close to their Haunted Gold Mine.

[ Click to Enlarge - 120 KB ] Two Archers harass an Undead player for a very long time.