This page has some information about defeating the vile and annoying Humans! It is not complete so it has some sample tips.

Rush!
Humans are weak to early rushes, especially if they don't have an Arcane Tower. Focus all your attention on killing their Peasants or Militia. Ignore the Footmen and Heroes because your goal is to kill their Peasants to get them behind. Good human players can defeat this attack but even if they lose some (maybe 4-5) Peasants your attack can be successful if you don't lose a lot or any units. You can then come back later with a larger army to finish them off.

Go Anti-Magic
The Humans often focus on using Priests and Sorceresses. Go Anti-spellcasters to defeat them.

Kill the Archmage
Humans often rely on the Archmage for either Blizzard or Water Elementals. The Water Elementals are summonables so deal with them or kill the Archmage using Hero killers.

Scout for and stop early expansions
The sneaky Humans sometimes try to get an expansion at the start of the game. Patrol for this and hit it as soon as possible.