I think the best secondary class for Mandy is .Because there are so many team cards, such as Open Gate, Ward of Protection, Premonition and so on.Another mystic investigator in team doesn't matter, because he/she can take the upgrade one or some other mystic cards. Most of mystic investigators' deck size is 30, sometimes they can't take those cards. However, Mandy's deck could include 50 cards.

The deck

Now let's talk about the deck. Her ability is about "search", so we should take as many "search" cards as possible: Eureka!, Mr. "Rook", which are very useful cards, especially Mr. "Rook". In addition, Research Librarian is also useful. For one hand, it can help you "draw" your own weakness as early as possible so that it won't disturb other more effective "search" effect. The other hand, Occult Evidence and Astounding Revelation need some cards to trigger.

What can she do

The first thing she can do is investigation. Her base value is 5, which means even if doesn't have key cards in hand, she can also investigate using the basic "investigate" action. If she has some cards providing "search" ability, she can discover clues directly by her special cards Occult Evidence. This card means the same cost, an action for a clue(Although Mr. "Rook" is ability, you also need spend one action to shuffle it back), but you don't need any skill test, which means automatically success. So it's very useful. Besides, Occult Lexicon and "I've got a plan!" help her attack.