This works for Campaign mode and Conquer the realm mode. You can 
also use it for any other troops in order to have more than what 
the game allows you to have. Just make sure you have money to buy
another Colossus or more troops. As you can't have more than one,
transfer him from your campaign army to a zone you think is likely 
to be attacked by the enemy. Then buy another colossus. 

Next, go to the land where you transferred the first colossus and
(here we need luck/patience) wait for the enemy to attack you. 
Once battle begins, you have now 2 Colossi in your campaign army.
You can even have more if you have enough money. Just do not transfer
or buy/sell one after you have acquired them, or you will lose them. 
If everything goes right, Let the slaughter begin!