Go to the Multiplayer menu and pick the AI practice Game mode. You can pick any map but Battered Bluff is Rather easy, its small and can be won on quickly. You can pick any faction, but if you have played stroy mode Ironheade will be the easiest one to get used to, the other two factions have small differences that can make them a bit more complicated to use. Pick any opponent I prefer to fight against Ironheade. You can press X to change the difficulty if you want it to go quickly just use the "Most Gentle AI Player". When you Have everything set up just start the game.
Start out by creating some head bangers and a killmaster to heal them and send them out to the center fan point. When they knock the tower down, build a merch booth then leave them there to defend it, Create another squad of head bangers and a Killmaster and send them over to one of the side towers to take control of one of those points, After you take two towers start upgrading your stage as you are able to and replenish units when needed and defend your towers, after you have taken all of the middle towers build up your army for a final assault on the enemy stage. By this time you should at least have all of your headbangers and killmasters upgraded add in whatever other units you want and you should be able to take down the enemy within 10 to 15 minutes.
I used mainly Head bangers and metal beasts.