Survivor and Taiyo are right, vertex manipulation is annoying and I avoid it as much as possible.
However, personally, this is what I would do:
1. Clone the bottom brush in the image and place it on top of the original brush.
2. switch to side view
3. ctrl-drag the top edge to the right inclination.
4. 2-point clip/resize the rest of the brush to remove any empty spaces/overlaps between the column and the wall.
Alternately, you could take a squared-off version of the top column, split it into 3 in top view, ctrl-drag the exterior pieces, clip the edges that are sticking into the wall in side view, then merge the three brushes back together. (this may seem confusing, I'm not being very eloquent

)
This technique works quite well for me. And I'll let you in on one of my dirty secrets: I have never, ever, had to use 3 point clipping in any of my maps. Honestly, I don't even know how it works
I must be odd I guess
