Impact wrench or cheater pipe on a breaker bar and 6 point socket of correct size.
Hate to tell you, but getting the nut off is the easy part. Breaking the stump jumper loose from the tapered splines can be a bear. If you will put "stump jumper" in the search box at the top of the page you will find many different tips on getting it off. Most of the threads are about KK brand cutters