Thanks Ed for a fun Assassin!
I don't know whether I found the same way as Ed did to crack this puzzle; my key breakthrough was in step 12 (with hindsight it was possible earlier than that) and I used another interesting move in step 19 to get a quick finish.
Here is my walkthrough for A204. I forgot until the final stages that this is a Killer-X so if you work through my steps using a software solver in editor mode please switch off the Killer-X option.
Prelims
a) R1C67 = {12}
b) R1C89 = {18/27/36/45}, no 9
c) R23C7 = {18/27/36/45}, no 9
d) R5C12 = {18/27/36/45}, no 9
e) R56C5 = {29/38/47/56}, no 1
f) R56C8 = {12}
g) R67C1 = {18/27/36/45}, no 9
h) R9C56 = {69/78}
i) 10(3) cage in N1 = {127/136/145/235}, no 8,9
j) 21(3) cage at R3C9 = {489/579/678}, no 1,2,3
k) 9(3) cage in N7 = {126/135/234}, no 7,8,9
l) 19(3) cage in N9 = {289/379/469/478/568}, no 1
m) 27(4) cage at R6C2 = {3789/4689/5679}, no 1,2
n) 34(5) cage at R1C4 = {46789}, no 1,2,3,5
Steps resulting from Prelims
1a. Naked pair {12} in R1C67, locked for R1, clean-up: no 7,8 in R1C89
1b. Naked pair {12} in R56C8, locked for C8 and N6
1c. 34(5) cage at R1C4 = {46789}, CPE no 4,6,7,8,9 in R3C56
2. 45 rule on N3 2 innies R1C7 + R3C9 = 11 = [29], R1C6 = 1, clean-up: no 7 in R23C7
2a. 21(3) cage at R3C9 = {489/579}, no 6
2b. 10(3) cage in N1 = {127/136/145/235}
2c. 7 of {127} must be in R1C3 -> no 7 in R2C23
3. 20(5) cage at R2C5 = {23456} (only remaining combination), no 7,8,9
4. 45 rule on R9 4 innies R9C1234 = 11 = {1235}, locked for R9
4a. 4 in R9 only in 19(3) cage, locked for N9
4b. 19(3) cage = {469/478}
4c. 9(3) cage in N7 = {126/135/234}
4d. 4,6 of {126/234} must be in R8C1 -> no 2 in R8C1
5. 7,8 in N2 only in R1C45 + R23C4, locked for 34(5) cage at R1C4, no 7,8 in R3C3
5a. 45 rule on N2 2(1+1) outies R3C3 + R4C6 = 10 = [46/64], CPE no 4,6 in R4C3
6. 45 rule on N1 3 innies R3C123 = 17 = {368/458/467} (cannot be {278} because R3C3 only contains 4,6), no 1,2
6a. 45 rule on N1 2 outies R4C12 = 1 innie R3C3 + 7
6b. Min R3C3 = 4 -> min R4C12 = 11, no 1 in R4C12
7. R3C7 = 1 (hidden single in R3), R2C7 = 8
7a. 2 in R3 only in R3C56, locked for N2
8. R78C9 = {12} (hidden pair in C9)
8a. R6C9 + R7C8 = 12 = [39/48/57/75], no 6,8 in R6C9, no 3,6 in R7C8
9. 45 rule on N9 2(1+1) outies R6C9 + R8C6 = 12 = [39/48/57/75], no 2,3,4,6 in R8C6
10. 45 rule on C12 1 outie R8C3 = 1 innie R2C2 + 6 -> R2C2 = {123}, R8C3 = {789}
11. 45 rule on N6 1 remaining outie R5C6 = 1 innie R6C9, no 2,6,8,9 in R5C6
11a. 9 in N6 only in 30(5) cage at R4C7 = {34689/35679}
11b. 8 of {34689} must be in R4C8 -> no 4 in R4C8
12. 30(7) cage at R6C6 = {1234569/1234578}, R9C56 = {69/78}, CPE no 7,8,9 in R8C6 (R8C6 “sees” all 6,7,8,9 in the 30(7) cage and R9C56, which cannot both contain 6,9 and cannot both contain 7,8)
12a. R8C6 = 5
12b. R9C3 = 5 (only remaining place for 5 in 30(7) cage), clean-up: no 4 in R6C1
12c. R7C8 = 5 (hidden single in N9), R6C9 = 7 (step 8a), R5C6 = 7 (step 11), clean-up: no 4 in R1C9, no 5 in R45C9, no 2 in R5C12, no 4 in R56C5, no 2 in R7C1, no 8 in R9C5
13. Naked pair {48} in R45C9, locked for C9 and N6 -> R9C9 = 6, clean-up: no 3 in R1C8, no 9 in R9C56
14. R9C56 = [78]
14a. R9C7 = 4 (hidden single in C7), R9C8 = 9
14b. Naked pair {37} in R78C7, locked for C7 and N9 -> R8C8 = 8, clean-up: no 2 in R2C2 (step 10)
14c. R4C8 = 3 (hidden single in N6)
15. 9 in C6 only in R67C6, locked for 30(7) cage at R6C6, no 9 in R7C5 + R8C45
15a. 1 in 30(7) cage at R6C6 only in R7C5 + R8C45 + R9C4, locked for N8
16. 5 in N2 only in R23C5, locked for C5, clean-up: no 6 in R56C5
17. 10(3) cage in N1 = {127/136}, no 4, 1 locked for N1
17a. R3C123 (step 6) = {368/458} (cannot be {467} which clashes with 10(3) cage), no 7, 8 locked for R3, N1 and 24(4) cage at R3C1, no 8 in R4C12
18. Naked pair {467} in R3C348, locked for R3
19. 10(3) cage in N1 (step 17) = {136} (only remaining combination, cannot be {127} = [712] which clashes with R2C2 + R8C3 = [17], step 10), locked for N1, 6 locked for C3 -> R3C3 = 4
20. R2C1 = 2 (hidden single in N1), clean-up: no 7 in R7C1
20a. Naked pair {58} in R3C12, locked for R3, N1 and 24(4) cage at R3C1, no 5 in R4C12
21. Naked pair {79} in R1C12, locked for R1
21a. Naked pair {68} in R1C45, locked for R1 and N2 -> R1C3 = 3, R2C23 = [16], R8C3 = 7 (step 10), R1C89 = [45], R2C89 = [73], R3C8 = 6, R2C56 = [54], R4C6 = 6, R23C4 = [97], R78C7 = [73], clean-up: no 8 in R5C1
21b. Killer pair 2,3 in R3C5 and R56C5, locked for C5
22. R8C2 = 9 (hidden single in R8), R1C12 = [97]
23. R3C12 = {58} = 13 -> R4C12 = 11 = [74], R45C9 = [84], clean-up: no 5 in R5C12
24. 1,2 in R4 only in 16(4) cage = {1258} (only remaining combination) -> R5C4 = 8, R4C4 = 5, R5C7 = 9, R5C5 = 1, R5C3 = 2, R1C45 = [68], clean-up: no 1 in R5C1, no 3 in R56C5
24a. Naked pair {36} in R5C12, locked for R5 and N4 -> R56C7 = [56], clean-up: no 3,6 in R7C1
24b. Naked pair {29} in R56C5, locked for C5 and N5 -> R3C56 = [32], R6C6 = 3, R7C6 = 9
25. R6C4 = 4, R567C3 = {189} = 18 -> R7C4 = 3 (cage sum)
26. 27(4) cage at R6C2 = {5679} (only remaining combination) -> R6C2 = 5, R7C2 = 6
It was only at this stage, when I started checking whether I’d reached naked singles, that I saw at the top of my worksheet the note that this puzzle is a Killer-X. Up to this stage I haven’t made any eliminations resulting from placements on the diagonals.
So close to being a unique puzzle without using the diagonals!
27. R5C5 = 2 (hidden single on D\), R7C3 = 8 (hidden single on D/)
and the rest is naked singles.
Happy Christmas to all forum members!