Best tool for cutting a 2" horizonal sloped canal cut through a 11' plaster/lath internal wall
If you use a circular saw a straight pair of lines can be cut. Then a simple wood chisel with a sharp whack will remove the block to make your channel. Steel nail plates used for electrical will protect where the lines are trapped by the studs. In between the studs push the line set back an inch so distance will be protecting it if you run the line set with the drain. 

I would suggest your control wire for the wire beside unit be run in the channel with the drain. This way the line can be identified later with a stud finder with voltage alert. I agree with gravity drains as I have seen many pumps fail.