I have a bosh electric jack hammer,
and my Daughter was or could run it when she was 16, and she was not a big girl.
doing a narrow area like that is difficult, but if you can get a diamond blade and even jsut get an inch deep cut in the floor it will help tremendously, and it will make a neat look to the job, the deeper the cut the better, especially on some thing that narrow,
if the cut is not possible,
take a sharp bull point chisel, and make a series of dimples spaced about every 2" to 3" just drive the tip about no more than 1/2" deep along a line you want to have for the edges, then come back deepen them some, if you have a 3" bit, you can then place that on the dimples, and make a longer mark, and more than likely you will create a crack in the slab that will give you a place to keep it from breaking out more of the slab.
the saw cut is best tho,
on a slab start in one area about 6" to 8" around and work it in multiple areas, until it is pulverized, clean out the broke cement as you go if possible.
clean out as much broken cement as possible once you get a hole, then start and jsut take off a few inches at a time, unless the concrete has some place to go it is hard to break, up,
I will repeat again take small bits, and it should go fast,
If you cut the trench wider say 12" to 16" I think you will find it easier, unless you have deep cuts on each side,