Here's some other strategies I've found to work. There's also some tips.
How many Sappers does it take to seriously mess up someone's base?
Farm=1
Barracks=2
Lumber Mill=2
Blacksmith=2
Townhall=3 (with any upgrades=4)
Any Tower=1
Shipyard=2
Foundry=2
Refinary=2
Alchemist=2
Ogre Mound=2
Altair of Storms=2
Temple of the Damned=2
Dragon Roost=2
Invisibility is a very useful spell in Multiplayer Games.
Team it up with Blizzard for a mage bomb.
Make two mages, and wait till they're up to their maximum MP.
Research Blizzard and Invisibility.
Have one cast Invisibility on the other close enough to their base so the spell doesn't wear off, but far enough away so if they come at you with a few Bloodlusted Ogres, you can get away with your investment of mages.
Have the Invisible Mage cast Blizzard next to himself and in between their Gold Mine and their Town Hall.
Get the @#$% out of there.
Polymorph their biggest guys with your invisible Mage.
Bring your Invisible Mage into their base, and cast Flame Shield on one of their Peons. The Peon will kill many of their with the Flame Shield before the Player even notices.
Use Bloodlust. It is probably the best underused spell.
Use your Sappers wisely. They could quite possibly be the key to winning.