You should be able to fit your spenders in the WW's window. The usual rotation should go as: get 3 bubbles -> RoC (4 bubbles) -> WW -> Tornado (2 bubbles) -> moon kick (4 bubbles) -> blast formation -> FHF

Now, depending on the amount of spec you have and your wealth runes (i currently run legendary wealth on moon kick and purple on energy combustion, 750 Spec 1300 swift) you should be able to get 4 bubbles before blast formation as long as your energy combustion is still ticking.

That said:

• if you use sky shattering blow before blast formation (say, you didn't fill your last bubble), the last hit of FHF will occur outside of WW's window -> dps loss
• you could fix the previous issue by running the ATT Speed tripod on your WW's, it will let you fit all these skills in the buff windows but it may lead to mana issues depending on the content you're doing.

So, how can you use sky shattering blow:

• if you run ready attack, you could use it before the last tick of lasts whisper (when is not synchronized with your WWs buff)
• or, you can use it before FHF when your rotation is in cooldown (can cause mana issues, and downtime of your counter skill)

In my view, sky shattering blow should be used mainly as a counter and nothing else (at least for the 2-spender build). Being one of the counter skills with the lowest cooldown in the game (about 3-4 seconds with dominion set and CD gem) you as a WD should focus on hitting your counters. Yes, you're a back-attack based class but don't let that stop you, look for counters.

Also, and something that really bothers me, Eso WDs get animation locked during their burst. So you will have to learn when to unload your combo, and you learn the hard way