First you must be playing the new battle version of the game - go to "play" and if you are directed to a screen where you have to choose the format you want to play in from a drop box on the right, then you need to click on the link above that which says "switch to new game version"
From there, you can choose to play in solo mode.
If you are already using the new version, then you simply need to click on the icon that says "back to main menu" from the start-up screen on the battle screen and again, choose solo mode
1. press the game and go to menu if you haven't already.
2. Click single player
3. Press the little orange arrow to get the other solo missions
4. then click on omega team and buy it, note it does cost 20 credits.
5. Play Dont forget the special deck formats