How to skim coat a wall that receive a heavy light at night?
You should take note of Ed Beal's comment above. I am a bricklayer by trade and I have seen this same problem with wall mounted lights on brickwork a lot. The brickwork looks, no, IS spot on, but at night, it looks steppy because (most) bricks are not flat. 

I'll stick my neck out here and say that it's going to be very difficult to get a perfectly flat surface with cement based coatings/ rendering etc. i.e good enough to shine lights down. 

The answer is to do what the historic buildings do. Shine the light ON the building (not down it). Not only does this look spectacular, making the house stand out, it will of course totally 'hide' the walls you're not happy with. This usually means lamps mounted in the grounds/bollards etc. and angled back towards the walls.