Couldn't resist - I run into this problem all the time!
Kliment, I agree with your technique, with the following exception...
No need to save the stair separately. Just make the stair wide enough to reach across the stair well's widest point, do the solid element operation and leave it as-is. Just sayin'.
I haven't encountered this particular situation in AC12 yet. On AC9, I'd actually make a slab to use as the cut body, do the SEO, then place the slab on a 'scratch' layer I keep turned off (unless I need to see it for revision). That way, if the skewed wall moves, I can just edit the slab I used and it's up-to-date without any saving or reloading of library parts. This was necessary, of course, because in AC9 I couldn't SEO 'beyond' the back side of the wall. I know they've changed this vertically in AC12 (which is REALLY cool!! used it today!), but not sure if it works 'laterally' (didn't notice / try.. will have to look and see).
My two cents.