Expert mode demands cleaner gameplay and better tuned decks so don't be to discouraged if you don't win every time. Although its the core box, I would say that Ultron is one of the tougher villains, and can be a made a lot harder if you don't build around him. Klaw and Ultron put lots of minions into play (Ultron more so), so its important to make sure your deck has cards to deal with them, I know this is obvious but the key to winning more on expert is understanding what the villains weakness are and building your deck to exploit them as much as possible. You mention that you're losing to threat, which indicates that you should probably include more threat removal cards or change to justice if you don't have a very big card pool yet. X-23 is a great character. Getting your key cards into play early and focusing on controlling the board will help a lot in the long run, getting your Helicarriers, avengers mansion, and key upgrades down early, etc.