Short but good answer pls (100 words each if possible)

What are some of the ways you can challenge a contract's validity? If you were writing a contract, what steps would you take to ensure your contract is impenetrable?

Mark is the manager of Mayhem Enterprises. Mark wants to diversify Mayhem's assets, so he enlists one of his ironically un-angelic friends, Angel, to obtain inside information on the Stock Market. When he offers to pay Angel for this task, Angel tells him she is willing to do this as a favor because they are friends. After Angel gets the information Mark wants, however, she asks Mark for a share in the newly expanded company. Will be able to obtain her request? Why or why not?