ERP implementations are among the toughest IT projects, but that is no excuse: to not get the results you anticipated (and were promised) would, as Dolly Parton once said, "fry my grits"!
Truth and reality. The truth is that many consultants will say they like performance type contracts, but the reality is that most do them only when we are required to and very rarely if ever for IT integration projects. Why? They depend on too many factors outside of their control, and we consultants like controlling our own destiny (and other things as well). :)
You have two problems. One is the implementation, the other with the consultant. The first you can deal with, whether it is redoing the work, more / better training, etc. Check your contract to see if there are any clauses that would indicate that there was a certain standard that they were expected to meet.
The second, the relationship with the consultant, is more problematic. You must talk with them directly and let them know your feelings about the lack of performance as based on the contract and their promises, and figure out where you go from here. Converting to a performance-type contract may not work - the consultant won't want to do that without some incentive ($$$) to do this. However, if there is sufficient incentive, whether it is actually $$$ or the threat of a lawsuit, they might consider it.
Be sure to have the contract language regarding expected results and scoping documents handy, as they will refer to those in the negotiations.
Remember, YOU own the project and its direction, and they work for you. Good luck!