A124 Walkthrough - courtesy Afmob

1. C123
a) Outies C12 = 12(2) = {57} -> R9C2 = 5, R1C2 = 7
b) Cage sum: R1C3 = 1, R9C3 = 2
c) 13(3) <> 9
d) 14(2): R5C1 <> 9
e) Innies+Outies C1: -2 = R5C2 - R46C1 -> R46C1 <> 2 (IOU @ N4)
f) 2 locked in 13(3) @ C1 for N1 -> 13(3) = 2{38/56}
g) 16(3) <> {367} because (36) is a Killer pair of 13(3)
h) Innies C1 = 16(3) <> 9 because R5C1 = (568) and {169} blocked by Killer pair (19) of 16(3)
i) 9 locked in 16(3) @ C1 for N7 -> 16(3) = 9{16/34}
j) 7 locked in Innies C1 = 7{18/45} for N4 since {367} blocked by Killer pair (36) of 16(3)
k) Innies C1 = 16(3): R46C1 <> 5,8

2. C123 !
a) 7 locked in 21(3) @ N7 = {678} because 5,9 only possible @ R8C4 -> CPE: R8C1 <> 6,8
b) 14(2): R5C2 <> 8
c) R6C1 <> 1 since it sees all 1 of N7
d) ! Outies N7 = 17(2+1): R6C2 <> 1,8,9 since 10 <= R6C1+R8C4 <= 15
e) 18(4) = 7{128/146/236} because R6C1 = (47) -> R6C1 = 7
f) 1 locked in 17(4) @ N4 for R4 -> 17(4) = 1{268/349}
g) ! Killer pair (69) locked in 17(4) + R5C2 for C2
h) 18(4) = {1278} -> R6C2 = 2, R8C2 = 1, R7C2 = 8
i) 16(3) = {349} locked for C1
j) 13(3) = {256} locked for C1+N1
k) R5C1 = 8 -> R5C2 = 6
l) 21(3) = {678} -> R8C4 = 8

3. R12345
a) Innies R1234 = {89} locked for C5+45(9)
b) 19(3) = 9{37/46}
c) Grouped X-Wing 9 locked in 19(3) + R34C5 for R34
d) Hidden Single: R2C2 = 9 @ C2
e) 8 locked in 18(3) @ C3 = 8{37/46} -> R2C4 = (67)
f) 9 locked in 12(2) @ R5 = {39} locked for R5+N6
g) 9 locked in 11(2) @ N3 = {29} locked for R1+N3
h) 10(3) = 1{36/45} -> 1 locked for C9+N3

4. C789
a) Innies C89 = 15(2) = {69} -> R1C8 = 9, R9C8 = 6
b) Cage sum: R1C7 = 2, R9C7 = 9
c) R5C8 = 3, R5C9 = 9
d) Innies C9 = 12(2) = [48/75/84]
e) 17(4) = {1457} -> 7 locked for C8+N9; CPE: R4C8 <> 4,5
f) 14(3) @ N9 = {248} -> R9C9 = 8; 4 locked for C9+N9
g) 17(3) = {359} since R78C7 = (135) -> R8C6 = 9; {35} locked for C7+N9
h) 22(4) = {2578} -> R4C9 = 7, R4C8 = 2; {58} locked for C8+N3
i) 10(3) = {136} locked for N3
j) 14(3) = {347} because R23C7 = (47) -> R2C6 = 3; 4 locked for C7

5. R6789
a) R5C7 = 1, R6C8 = 4, R6C9 = 5
b) Innies R6789 = 9(2) = {36} locked for C5
c) 15(3) = {168} because R6C67 = (168) -> 1 locked for C6

6. N2
a) 18(4) = {1458} since R1C456 = (4568) -> R2C5 = 1, R1C6 = 8; {45} locked for R1+N2
b) R2C9 = 6, R2C4 = 7
c) 18(3) = {378} -> R2C3 = 8, R3C3 = 3

7. Rest is singles.

Rating: 1.0. I used Killer pairs and some Outies analysis.