Sorry to be a little late on this but I've been labbing SS hitstun cancel vs Boost Kick and found some really useful stuff. It's always been WAY harder to cancel than Shuttle Loop for me, and despite me being against reliance on the SS hitstun cancel in general (as many of you know; I still think it will be patched out), I realise that being able to consistently escape ZSS' most dangerous kill move (not to mention being the only character to be able to do so) would be amazing. I don't know if my findings will be news to any of you or if I'm just late to the party, but I'll post all my findings nonetheless.
First off, as we all know, SS hitstun cancel can only be used to cancel light hitstun. The only hit of ZSS' Up B that causes light hitstun is hit number 6. This is the hit where you're above her that causes semispike knockback which brings you down in front of her to be hit by hit 7 (stalls your position) and hit 8 (the knockback inducing kick). We CANNOT escape on any other hit. So the first step to getting a consistent cancel is understanding when and where we can actually get out.
Now to get out of Shuttle Loop, most of us will just mash Side B. This works really well for SL for two reasons. One, the time between SL hit 1 and hit 2 is (much) longer than BK hit 6 and hit 7. Secondly, SL has very fast startup and MK can vary when he actually uses the move to launch you off the top during his combo, making it hard to SS out on reaction. With ZSS however, BK is multi-hit which makes you suffer repeated hitlag and thus it's MUCH easier to react to getting hit by it, either raw or combo'd into. Also, she cannot vary the timing with her true combos into it like MK can (I'm not going into airdodge traps, they are not as dangerous as a guaranteed death combo no matter how you slice it). She will either hit you with Uair Uair Up B, or Uair Up B depending on percents and positioning. So reacting to Boost Kick on hit is very doable especially with the combo everyone's talking about right now: Dthrow Uair Uair Up B at ~40%.
So, how do we actually get out? Well first off, mashing side B against BK is a horrible idea. This is where most of us including myself we're going wrong. This is because the multi hit properties will cancel out our repeated attempts to Shadow Sneak and our only chance to escape will be using it right after hit 5 which is a really small window. Using it too late on hit 6 will usually get us hit by hit 7, because SS has startup before Greninja disappears.
What we can do instead is abuse the Sm4sh buffer to consistently escape. We can actually buffer SS out of hitstun, which I didn't know about until I tested this. So, if we SS at any time between hit 4 and hit 6 and just press the button ONCE, we will get out of BK scot-free Every. Single. Time. It's amazing. I was able to get Greninja up to over 200% with this, cancelling out of BK about 20 times in a row. We basically have a window of time equal to the size of the buffer window (which I think is about 15 frames, I'm almost positive it's larger than Brawl's 10f buffer) to escape this move. It doesn't matter what direction you SS in. Just press side B once any time between hit 4 and hit 6, and you will get out.
The timing to escape can vary a bit because when you get combo'd into BK in the air she will usually whiff the first hit and can whiff the 2nd as well, which changes the timing. Pay close attention to what part of the kick she actually hits you with (it will usually be hit 2; hitting later will usually cause you to pop out of the move anyway). With BK OoS, she will always hit you with hit 1 so just use the standard timing.
For BK OoS, we land before ZSS and can get a guaranteed (turnaround) jab or Dtilt punish. And yes, in a cruel twist of irony we will be so close to her that Dtilt to JC Usmash will true combo if she doesn't DI away. I haven't tested the true punishes on a high up BK, I will have a look tonight. I think we'll probably get nothing but meh, not dying is reward enough.
tl;dr: Don't mash Side B out of Boost Kick, press it ONCE between hit 4 and hit 6 and you will get the SS hitstun cancel every time due to the buffer.
I had written SS out of BK off as inconsistent jank, as I think many of us did. But, we were very wrong. This is a consistent escape that can be done very easily on reaction because the window is so large, no matter if ZSS raws or combos into BK. Greninja is the only character in the game who can escape ZSS' death combos with 100% consistency.
I think this has a significant effect on the MU, am happy to list it as even, and would like to explore the possibility of us beating ZSS.
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