I've tried this a couple of times:
I made a deck consisting of 40 non-land cards. When I first did this I started out with creatures only. During what would be my Phantom Opponent's (PO) main step I roll a D6. On a 1, 2 & 3 there was no effect. But on a 4, 5 & 6 I would roll the D6 again to determine how many <X> permanents I could put into play from the PO's deck.
Rolling a 4 meant I would roll the D6 again and put in <X> minus 3 permanents into play from the PO's deck. On a 5 I would roll the D6 and put in <X> minus 2 permanents into play. And a 6 <X> minus 1 permanents into play.
Each permanent is put on the stack before it resolves so you can respond to it.
In response to any spell I play my Phantom Opponent rolls a D6:
on a...
1-No Response
2-Counterspell
3-No Response
4-Mana Leak
5-No Response
6-Force Spike
The PO gets every step except he doesn't draw a card. Obviously there is no element of skill on the PO's part but if your looking to just play mechanically this might be a way to go.
I've also have played this with two Phantom Decks to resemble multi-player. You can adjust the rules anyway the suits you. For example; when I'm playing a deck with propaganda I roll the D6 for each creature that could attack me and each propaganda I have in play. Each pip on the dice is one mana.
I haven't figured out how I would adjust it to playing land destruction spells, it'll be hard to figure that one out.
~Mark