Idea 1:
Put some wide masking tape covering where you want the cut to be on
the top of the counter top. Turn the counter over and mount a
straight on the back. Use a quality fine tooth blade in your skill
saw. After you made the initial cut in the counter top come back and
finish the cut in the back splash.

Any cut you have to clean up with a sander won't be very tight.

Idea 2:
Cut the counter long then clean up with a router, flush cut bit and a
straigt edge.

Idea 3:
Personally I would use the Radial arm saw and six pack advice. Where
do you live? (I like micro brews!)