There are plenty of ways to tackle the job you both are going after. I would suggest a 4 1/2" Grinder with a Mortar Wheel and a Diamond Blade.
Both will cut through the mortar (Usually put on in two coats 1)Scratch Coat 2) Brown Coat), which is said to be far more water repellant and moisture resistant than Hardyboard.
This will not tax the adjacent plaster walls!
For wire and wood try the grinder with a metal blade or a reciprocating saw with a bimetal blade.
Carefull, both will cut through your plumbing lines in a flash.
I also recommend having a very good shop vac running to collect dust and small debris.