I've had to do about a million of these. They are a lot of variables to any of these. If what I see can be relied on in the photo, the slope is favoring the weight of the tree. Brush off the dirt near that knot closest to the ball in your pic. Take your 36" bar and make a downward cut. Insert a wedge when about halfway through. Finish the cut. Whether the root ball moves or not is not going to effect where you are standing (highest part of the slope). If you're concerned about saw pinching, you can always shore up the tree just before the cut with a rock or couple of 6x6 dimensional chunks. If none of those were available, we'd make chunks from the stem and jamb them under the tree. You need only do this enough so your saw isn't jammed to the ground.You cut these things with two two thoughts in mind:1. a short enough stump from the ball so as for it to not act as a baseball bat for the cutter should the ball rotate and 2. To leave the stump long enough to give the root ball enough support so it does not tumble forward toward the cutter. I'd suggest making the separation cut first because what you do not want to have happen if you are trimming the tree first is to keep thinking what the tree will do when you finally make the separation cut. This thought will act as a distraction while holding a saw in your hands and that's never good with a running saw in your hands. Get it over with right from the beginning.
