I'm afraid the difficulty of this one isn't that high. You'd think for such a special number the Times would give us a really special puzzle but they didn't.
However it's probably not fair for me to assess this puzzle given my experience in this variant.
I suggest you to try it again once more, and if still no success post your stuck state here so I can tell you which move you missed.
I've had a couple of goes first time round I focused on clashes in the 19(3) and 21(3) cages on boxes 2 and 3.
Next time round again focusing on boxes 1-3 I found the innies of Box 1 useful. The single innie you can obtain in box 8 is also a big help with the 11's.
BTW The SS score is 0.87. So lower than Assassin level which is the case for most Times puzzles.
Thanks, guys. All happy now. Regardless of Glyn 's ratings, to my mind this is by far the hardest Deadly that The Times has published in its paper. I cannot answer for the website.