Considering that Syndra has 4-5 spheres in the field then she casts ult on Sivir.
Unleashed Power has a brief cast time before it activates, but Syndra does not have to remain in place while the spheres apply their damage.
To quote another from http://leagueoflegends.wikia.com/wiki/Syndra#Unleashed_Power
Spell shields will only block the damage of a single sphere.
I want to know the difference if Sivir managed to cast the spellshield before Syndra casts her ultimate and after.
Before: Will the damage of the entire ultimate get blocked? Or just the first sphere?
After: Will it still block a single sphere when it was already casted (since Zed/Zilean's time-delayed damage procs even after the spellshield was casted)?