What I need to trim down is the 2x4 stud that comprises the bottom part of the frame for the window to be level with the cabinet tops. I thought about re-attaching the cabinets 1/2" higher, but that effectively necessitates moving all the cabs in the kitchen higher which is not acceptable. I will look into an oscillating tool. Is a belt sander an option to remove this much material?
Studs are the vertical pieces that the wall board is nailed to. They are not under the windows, those are the sills on the outside and stools on the inside:
If I understand your question the existing cabinets including the backer board are level, BUT the windows stools are not? That's a huge problem. You don't want to lower the cabinets, and you don't want to remove, raise and level the windows? You need to start at the lowest point and determine the amount you have to work with.
I am a little familiar with this type of issue since I needed to add a 1/2" of hardboard on all my base cabinets so the stove top would rest on top of the solid surface "granite". You seem to have the opposite issue. Your cabinets are already too high/tall?
How thick is the concrete going to be? Can this be changed?
Are the "2 X 4 studs" you refer to cross braces /supports in the cabinet interiors?
Please give us a more detailed description, even a sketch of the problem would help a great deal. Removing large amounts off a 2 X 4 will not be easy by sanding. Cutting or planing is the easiest, cleanest and fastest method. The multi-tool is loud and slow, at least both of the ones I own, a Rigid and a Harbor Freight. A hand held power planer will remove a significant amount of material easily, but will blow small chips all over. Pick your poison, in this case.