DFG + Ult can't 100-0 you unless she's very fed. Yes, it can take a good portion of your health away, but it won't one shot you. Just be cautious when you're half health or lower - if she always has 1 sphere down, she can easily cast another to make it a 5 ult combo which can be pretty devastating.
I'd also like to mention that Syndra has very low mobility - if you play a champion with a quick gap closer, you simply have to wait until she misses her stun and then engage.
And to add on what already has been said: dark sphere has a delay time. With quick enough reactions, you can dodge most of the spheres she casts (boots help with this but aren't needed - assassin champions playing mid have no problems dodging my spheres). Repeatedly casting Q can also starve Syndra of mana early game, so use that to your advantage.