This is pretty simple. Set up a game with only humans, you can use one controller to control all players, locally.
When the world is populated make sure your enemies attack each other and you focus on building up small pockets of large troop quantities. Once you get a couple of 10 or more troops, just start rolling over enemies that have only one troop. When you get low on troops quit the attack phase and wait til the next round to build up your troops again.
The key thing is to keep your enemies attacking each other so their units drop to one and then you can vanquish them!
In the pic below, I setup a manual conquest game and placed 1 troop in most every region except two of mine in red (Africa and South America), and the blue and green 22 units top left. when the game starts I have the 22 units attack each other until they deplete and I roll into North America and the Middle East, Europe and Asia.
As you gain more cards, trade them in for more troops and build up your forces to mow more people down :)