And now for the last of the three V1 Assassins in the A160 to A165 range that I got stuck on at the time.
I'm a bit surprised that nobody has mentioned the first key breakthrough step for A165, the CPE resulting from the 4(2) cage at R4C7 being a naked pair. That turned out to be the reason why I'd got stuck. Once I'd found the part of it that I'd missed before the rest of the puzzle was straightforward; I'd already found the 45 in step 14a but it wasn't much help without that CPE applying to R5C8. For that reason I felt that Afmob's step 2c was also worth a !
Here is my walkthrough for A165.
Prelims
a) 11(2) cage at R3C3 = {29/38/47/56}, no 1
b) 6(2) cage at R3C7 = {15/24}
c) 13(2) cage at R4C3 = {49/58/67}, no 1,2,3
d) 4(2) cage at R4C7 = {13}
e) R56C1 = {16/25/34}, no 7,8,9
f) R56C3 = {16/25/34}, no 7,8,9
g) R56C7 = {17/26/35}, no 4,8,9
h) R56C9 = {79}
h) 13(2) cage at R6C4 = {49/58/67}, no 1,2,3
i) 11(2) cage at R6C6 = {29/38/47/56}, no 1
j) R78C4 = {18/27/36/45}, no 9
k) R78C6 = {15/24}
l) 11(2) cage in N7 = {29/38/47/56}, no 1
m) 13(2) cage in N9 = {49/58/67}, no 1,2,3
n) 8(3) cage in N1 = {125/134}
o) R345C2 = {489/579/678}, no 1,2,3
p) 28(4) cage at R1C6 = {4789/5689}, no 1,2,3
1. Immediate follow-up steps
1a. 8(3) cage in N1 = {125/134}, 1 locked for N1
1b. Naked pair {13} in 4(2) cage at R4C7, CPE no 1,3 in R4C456 + R5C78 (see comment before step 14), clean-up: no 8 in R3C3, no 5 in R3C7, no 5,7 in R6C7
1c. Naked pair {79} in R56C9, locked for C9 and N6, clean-up: no 1 in R6C7, no 4,6 in R8C8
1d. 28(4) cage at R1C6 = {4789/5689}, CPE no 8,9 in R1C45
1e. R78C4 = {18/27/36} (cannot be {45} which clashes with R78C6, no 4,5
1f. 4,8 in N6 only in R456C8 + R4C9, CPE no 4,8 in R1C8
2. 45 rule on D\ 1 innie R5C5 = 1 outie R2C3 + 2, no 1,2,8,9 in R5C5
2a. 1 on D\ locked in R1C1 + R2C2, locked for N1, clean-up: no 3 in R5C5
3. 45 rule on D/ 1 outie R2C7 = 1 innie R5C5 + 3, R2C7 = {789}, no 7 in R5C5, clean-up: no 5 in R2C3 (step 2)
4. 8(3) cage in N1 = {125/134}
4a. 2 of {125} must be in R2C3 -> no 2 in R1C1 + R2C2
5. 45 rule on D\ 3 innies R1C1 + R2C2 + R5C5 = 10 = {136/145}
5a. 11(2) cage at R3C3 = {29/38/47} (cannot be {56} which clashes with R1C1 + R2C2 + R5C5), no 5,6
5b. 11(2) cage at R6C6 = {29/38/47} (cannot be {56} which clashes with R1C1 + R2C2 + R5C5), no 5,6
5c. Killer triple 2,3,4 in R1C1 + R2C2 + R5C5, 11(2) cage at R3C3 and 11(2) cage at R6C6, locked for D\, clean-up: no 9 in R8C8
5d. 6 on D\ locked in R5C5 + R9C9, CPE no 6 in R9C5
[I originally started step 5 with
2 on D\ locked in 11(2) cage at R3C3 or 11(2) cage at R6C6 -> one of these cages must be {29}, 9 locked for D\, clean-up: no 4 in R9C9
This uses locking cages which is a type of combined cage.]
6. 1 on D/ locked in R1C9 + R2C8 + R3C7, locked for N3
7. 45 rule on C12 1 outie R2C3 = 1 innie R9C2, R9C2 = {234}
8. 45 rule on C89 1 outie R2C7 = 1 innie R9C8 + 6, R9C8 = {123}
9. Killer triple 4,5,6 in 6(2) cage at R3C7, R5C5 and 13(2) cage at R6C4, locked for D/, clean-up: no 7 in 11(2) cage in N7
10. 45 rule on D/ 3 innies R1C9 + R2C8 + R5C5 = 15 = {159/168/249/267/348/357} (cannot be {258/456} which clash with 6(2) cage)
10a. 7,9 of {249/267} must be in R2C8 -> no 2 in R2C8
11. R345C8 = {138/147/156/246/345} (cannot be {129} which clashes with R456C7 ALS block, cannot be {237} which clashes with R56C7), no 9
11a. 8 of {138} must be in R5C8 -> no 8 in R34C8
12. 6(2) cage at R3C7 = {15/24}, 11(2) cage in N7 = {29/38} -> combined cage R3C7 + R4C6 + R8C2 + R9C1 = 17 = {1529/1538/2438}
12a. 13(2) cage at R6C4 = {49/67} (cannot be {58} which clashes with combined cage), no 5,8
12b. R1C9 + R2C8 + R5C5 (step 10) = {159/249/267/348} (cannot be {168/357} which clash with combined cage)
12c. 1 of {159} must be in R1C9 -> no 1 in R2C8
13. 5 on D/ only in R4C6 + R5C5, locked for N5, clean-up: no 8 in R4C3
[When I first did the CPE in step 1b, I only saw the more obvious R4C6 and R5C7 and overlooked that it also applied to R4C45 and R5C8. No 1,3 in R5C8 makes a big difference to the next step; without those eliminations this puzzle I think would be extremely difficult to solve.]
14. Hidden killer pair 1,3 in R5C13 and R5C6 for R5, R5C6 = {13} -> R5C13 must contain one of 1,3
14a. 45 rule on R6789 4 outies R5C1379 = 20 = {1469/2369} (cannot be {1379} because 7,9 only in R5C9, cannot be {2459/2567} which don’t contain 1 or 3, cannot be {3467} because the 7(2) cages in N4 cannot both be {34}), no 5 -> R5C9 = 9, R6C9 = 7, 6 locked in R5C137, locked for R5, clean-up: no 4 in R2C3 (step 2), no 9 in R2C7 (step 3), no 4,7 in R4C3, no 2 in R6C1, no 2 in R6C3, no 3 in R6C7, no 6 in R7C3, no 4 in R7C7
15. Naked pair {26} in R56C7, locked for C7 and N6, clean-up: no 4 in R4C6, no 9 in R6C6
15a. Killer pair 2,5 in R4C6 and R78C6, locked for C6, clean-up: no 9 in R7C7
15b. 5 in N6 only in R456C8 + R4C9, CPE no 5 in R1C8
16. 11(2) cage at R3C7 = {29} (hidden pair on D\), CPE no 2,9 in R3C4 + R4C3, clean-up: no 4 in R5C4
17. R345C8 (step 11) = {138/147/156/345} (cannot be {246} because 2,6 only in R3C8), no 2
18. R1C1 + R2C2 + R5C5 (step 5) = {145} (only remaining combination), locked for D\, clean-up: no 7 in R7C7, no 8 in 13(2) cage in N9
18a. R8C8 = 7, R9C9 = 6, clean-up: no 2 in R7C4
19. R6C4 = 6, R7C3 = 7 (hidden pair on D/), R56C7 = [62], clean-up: no 1 in R56C1, no 1 in R56C3, no 3 in R7C4, no 2,3 in R8C4
19a. R5C6 = 1 (hidden single in R5) R4C7 = 3, R7C7 = 8, R6C6 = 3, R78C4 = [18], R2C7 = 7, R5C5 = 4 (step 3), placed for both diagonals, R2C3 = 2 (step 2), R3C3 = 9, R4C4 = 2, R3C7 = 1, R4C6 = 5, R5C3 = 3, R6C3 = 4, R56C1 = [25], R1C1 = 1, R2C2 = 5, R4C3 = 6, R5C4 = 7, R1C3 = 8, R5C2 = 8, R5C8 = 5, clean-up: no 9 in R8C2, no 3 in R9C1
20. Naked pair {79} in R4C12, locked for R4 and N4 -> R4C5 = 8, R6C2 = 1, R6C5 = 9, R6C8 = 8
21. R2C7 = 7 -> R1C9 + R2C8 = 11 -> R1C9 = 2, R2C8 = 9, both placed for D/, R8C2 = 3, R9C1 = 8
22. Naked pair {24} in R78C6, locked for C6 and N8
23. R1C6 = 9 (hidden single in R1), R9C6 = 7
23a. Naked triple {356} in R789C5, locked for C5 and N8 -> R123C5 = [712], R9C4 = 9
24. R1C6 = 9, R23C6 = {68} = 14 -> R1C7 = 5 (cage sum), R89C7 = [94], R9C2 = 2
25. R9C6 = 7, R89C7 = [94] = 13 -> R9C8 = 1 (cage sum), R4C8 = 4, R3C8 = 3 (step 17), R1C8 = 6, R1C2 = 4
26. R5C2 = 8 -> R34C2 = 13 = [67]
and the rest is naked singles.