While MN is an At Will state, workers still have a few avenues. Sex, race, religion, age discrimination can all be ways to sue an employer. Proving some of these might be a challenge.
As a worker, you also have the option of Arbitration. It would be conducted by a member of the National Mediation and Arbitration Boards. Those that choose this process, will be required to cover 50% of the cost of the Arbitrator, as well as other associated expenses. I've dealt with many cases in arbitration being a past union representative for the IAMAW. The process for an individual, is quite simlar, and you don't have to be in a "unionized shop", to file for an arbitration case.