
With the changes to Assassins, Syndra has become one of the best 100-0 champions there is. You need to be able to avoid her damage but keeping out of her short range. She will try to boost her range with her combo abilities, costing her mana. She'll only ever close in for an ult-kill. You need to have at least Zhonya's and something else to avoid her ult damage.
Syndra doesn't do well against champions she can't even get close to, like Lux. Alternatively, someone like Lissandra is 1 for 1 against Syndra. Longer Range poke really keeps Syndra controlled, however, it leaves you vulnerable should you get ganked since long ranged ability focused champions tend not to have a lot of mobility.