1 Attack early
If the computer AI was allowed to collect resources it will become very strong that stopping it might not be easy. Attacking the computer as early as in the feudal age can help. See how to do a feudal rush.
2 Attack with buildings
Even in the feudal age the computer AI can provide considerable resistance. To make sure you weaken it build many towers in one spot near its base. 6-8 towers might be needed to ensure proper defense. Fill the towers with as many archers are you can.
3 Build towers near the gold
The computer can't function or fight back without Gold. Build your towers near the Gold the computer is using.
4 Build a castle there
Build a castle to support your towers as the computer might fight back fiercely to to destroy the towers. Make the murder holes upgrade and Garrison many archers inside the castle.
5 Attack the villagers
Send some knights to prevent the computer from collecting Gold from another source. As soon as the computer's forces reach you retreat to your new base.
6 Create an army behind your new base
As you keep killing villagers with few knights make sure you prepare an army behind your new base.
7 Attack when the army is ready
Once you believe you have enough men , depending on how the computer's base looks like, you should attack.
8 No need to defend home base
When the computer AI finds a new base near its home base it never bypasses it to attack your original home base. All the attacks will go directly to the new base you built.