At the start of any random map game, at least one CPU opponent will become your ally if you give them 100 of the resource that they request. Remember the opponent that asked for the resources, as this will be important later. Wait at least until the castle age, then give that opponent the resources they requested. At this point during the game, you should have either well over 4,000 of all resources or a huge military (depending on your population limit). Either way, that payment should be no more than a small investment. Afterwards, set your stance with them to ally. When you have a large enough army, switch your stance to either enemy or neutral and attack them. They should be almost completely eliminated before they switch their own stance to enemy.