To maximize your ultimate's damage use it when there are many spheres on the battlefield.
After you hit an enemy champion with Force of Will follow up with Dark Sphere, they'll be slowed and have a hard time avoiding it.
Get one of your spells to rank 5 as quickly as possible, they gain powerful bonus effects at max rank.
Get boots early when playing against Syndra, they'll help you avoid many of her spells.
Attack Syndra after she's used Scatter the Weak, it has a long cooldown so she'll be vulnerable.
At max rank Syndra's spells gain bonus effects and change in appearance.