I'm dubbing this system "Magurbating" because it's not as good as the real thing but it gets the job done.  
  

The goal is to create an artificial opponent to play against. Obviously I can't replicate every single deck archetype so I'm focusing on creating a slow drip of creatures and turning them sideways with some randomization to simulate the unpredictable nature of the game.  
  

I've created a 30 card assortment of 2/2, 3/3, and 5/5 creature cards, Sorcery and Combat trigger cards. The way it works is after you complete your turn, draw a card from the Test Deck. If it is a creature card, play it to the battlefield. If it is a sorcery card, roll a D20 and resolve the result. If it is a combat card, roll a D20, resolve the result, and attack with all Test Deck creatures. I've divided the cards into three piles and colored their card type box as a method to create mini decks that range from easy to difficult (green/easy> white/normal> red/hard). You can use all three colors, remove any you deem too difficult or easy, order them in a specific layout for consistency or shuffle them all for a more chaotic gameplay experience.  
  
I treat it like a normal match and try to reduce the artificial opponent to zero. Since the deck is only 30 cards you can opt to end the game when the artificial opponent decks out or simply shuffle the deck and proceed in a more chaotic mode.
  
  
I don't think this system is perfect but I did a couple of rounds and it functioned well for what it was, I was thinking about adding a separate mini deck for instants that trigger depending on dice rolls at the start of each of my phases. Any input on this would be greatly appreciated!

I’m working on a goldfishing deck and could use some feedback or recommendations to improve it!