pnm wrote:
possibly considerably more difficult that the previous one?
...
Well "placed" among the "difficult" but not much more difficult than the previous one, in my opinion. The three rightmost columns (along with the three bottom columns give a lot of information), the 6x6 up and left area is the last part solved in the process.
Two general comments:
Of course these "killer's" can all be solved using only FAS (Full Analytical Solution) (any TAE, Trial and Error, possible to solve them of course, would be the consequence of not having applied correctly the "addition rules").
A very difficult "killer" could be conceived, designed, but always a corresponding very difficult 9x9 Calcudoku will be of a much higher difficulty. This advanced conclusion, after some time, could be confirmed just by relating statistically the number of solvers of both types.