While you are in a custom game, build sappers to blow 
up their farms, but don't let them come close after 
eachother, because they will blow eachother up. 

Train a mage or deathknight and research blizzard or 
death and decay. Sneak into their base (or run into-) 
and use the spell on the peons'gold route. (The way between 
the town hall and the gold mine) They will have to build new 
peons, wich gives you time to build a army. 

Research healing for the paladins. This is the strongest magic i
n warcraft. 

Army: First line = paladins with healing 
Second line = archers with longbowupgrades 
third line = balista's 

Hint: let your katapults atack ground. 

Placing your long range atack units in 'corners' will help. 
Submitted by ruurdjan