How can I trim 1/8 off the bottom of an installed cabinet?
With my interpretation of your circumstances, I feel the tool for this task would be an oscillating multi-tool, found just about everywhere.



The attached blade in the photo may be the one you'd use, although the semi-circular one would give you a smoother entry. They cut relatively slowly allowing for precise control. You'd be able to build a shim under the cabinet to keep the blade in the desired position vertically as you run it over the areas to be cut.

Depending on the clearance you have, you should be able to get all the way to the corners (the blade can be mounted in various angles) or have to take the chisel to break the hold-outs clear.