Indeed it was. Took ages to find the right nut-cracker to use - lots of nooks and crannies to search through. Very enjoyable. A walk-through follows - but if you want just a hint on the nook to start with - in tiny text here 4 innies of N89a tough nut to crack.
Assassin 16 –
Step 1
“45” on N89 -> 4 innies = 29
Max r7c45 + r9c4 = 7+8+9 = 24 -> min r7c9 = 5
->r67c9 = [15] and r7c45 & r9c4 = {789} only -> no 7,8 or 9 elsewhere in N8
“45” on N89 -> 2 outies = 10 -> r9c3 = {678} r6c4 = {234}
Step 2
“45” on c89 -> 2 innies = 3 -> r47c8 = [21]
“45” on r12 -> 2 innies = 3 -> r2c47 = [21]
r3c9 = 2 (hidden single r3)
9(2) cage in N3 {36} only -> no 3 or 6 elsewhere in N3 or c9
->14(2) cage in N23 = {59} or [68]
Step 3
“45” on r6789 -> 4 innies = 29 -> r6c5678 = {5789} only -> no 5,7,8 or 9 in r6c1234
->17(3) cage in N56 = {359/458} (not 368/467 since 3,4 & 6 only in r5c7)
-> 17(3) cage = 5{39/48} with r5c7 = {34} only, r6c67 = {589} with 5 locked in r6-> no 5 elsewhere in r6
“45” on N36 -> 2 outies = 13 -> r16c6 = [58] ([85] not possible since 8 has been eliminated from r1c6 in step 2), r16c7 = [95], r5c7 = 4, r34c7 = 13 = [76], r5c8 = 3 (hidden single N6), rest of 19(3) cage {79}
12(2) cage in N3 = {48} only -> no 4 or 8 elsewhere in N3 or c8 -> r3c8 = 5, r4c9 = 8
Step 4
“45” on r89 -> 2 innies = 5 -> r8c36 each max 4
-> 5 in N7 locked in 13(2) cage = {58} -> no 5 or 8 elsewhere in N7 or c1
-> r9c34 = [69/78] -> 7 in N8 only in r 7 (no 7 elsewhere in N8 - see step 1)-> no 7 elsewhere in r7
9(3) cage in N47 now {234} only
-> 6 in r6 now locked in 18(4) cage in N47 ->18(4) cage {1269} only
-> r78c3 = [91], r7c6 = 6 (hidden single r7), -> rest of 14(4) cage in N89 = 7 -> r7c7 = 3, r8c6 = 4, r89c7 = {28} = 10 -> r9c6 = 1, r9c9 = 4 (hidden single N9)
Step 5
r7c12 = {24} = 6 -> r6c1 = 3, r6c4 = 4
10(2) cage in N7 = {37} only -> no 3 or 7 elsewhere in N7 or c2 -> r9c34 = [69]
2 in N5 only in 29(5) cage -> no 1 in 29(5) cage (since the last 3 cells in that cage could not total 26) -> r4c4 = 1 (hidden single N5),
“45” on N5 -> r4c6 = 3
the rest is pretty straightforward
Edited again to make some points clearer (thanks again Andrew) and fix more typos