Don't rush, work methodically and precisely. Take care and pay attention. Sharp, clean blades on wet glass. Move the blade in 1 direction as the reverse move can trap grit beneath the blade and scratch the window.
Point out scratches before you clean the glass and document it.
Charge 3 - 5 times what you would normally charge depending on the amount of muck on the glass.
A limescale remover or white vinegar will soften mortar before scrapage.
Don't be scared of it, use your common sense, if you don't try it you'll never find out.