The best tool depends on how strong and hard the rock is. I had a similar situation where I had to install approximately 25 feet of 4 inch diameter drain line through granite, which is very hard and very strong. The method I used to chip the stone was to use an electric hammer with a pointed bit to "soften" up the rock along a line, then a flat "spade" bit to remove the rock. Very hard work, took a long time, but it worked.
If you have unlimited funds, an air trac is the best, but you will need to hire that out, can't rent that tool so far as I know. If you had a really large project, you would use explosives, but for your job that would not be cost effective.
If the rock is soft, such as shale, soft limestone, or siltstone, you could do the work with a star drill by hand.
There are unusual techniques that sometimes work, such as drilling small, closely spaced holes into the rock using a hammer drill, then filling the holes with expansive cement (Google expansive rock breaking). This works fine with certain types of rock, and is relatively easy, assuming you rent an adequately sized hammer drill.
