Despite being immediately nerfed by Riot Games by increasing her energy from 40 to 50 in patch 10.14, Syndra still proves herself to be the “ultimate sorceress” in the meta with her incredibly overwhelming power.
Here are some ways to help you counter or at least mitigate the strength of Syndra:
Activate Mystic

The simplest way to mitigate Syndra’s damage output is to activate the passive of Mystic, which adds magic resistance to the entire team. With 2 or 4 Mystic champions, in at least the first skill usage, Syndra will not be able to oneshot one of your champions. Moreover, with the mechanic of targeting the enemy with the highest health, Syndra will definitely not continue to deal damage to the first champion she has already used her skill on, helping your carry to survive much longer.
Currently, Mystic champions also have very strong skills and activations like Soraka (Healing + Celestial Guardian), Lulu (hex + Cosmos), Karma (buff + Dark Star),… so there’s no worry that these champions are just there to activate Mystic.
Silent Cloak

A rather simple item but can slow down the first skill usage of most enemy teams. With this item, your team can focus damage and take down some enemy champions before they even have a chance to use their skills.
Claw of the Trap

This is the best item to stop champions who can use skills in succession like Darius, Irelia,… or champions who spam skills like Ziggs and Syndra. When Syndra uses her skill on Claw of the Trap, she will be stunned for 4 seconds, effectively disabling the main damage source of this Celestial Guardian team for a long time during fights. However, this powerful item is still underrated in many servers around the world.
Wind Sword

Historically, one of the simplest ways to counter an overly strong individual in any comp is to use Wind Sword. If the target isn’t Aurelion Sol or does not have the Cloak of Disruption, they will be knocked up for 5 seconds at the start of the fight, significantly blocking the damage source of the enemy’s carry.
Irelia + Talon the Scout

In the Super Technology comp, when Irelia also activates the Demon Hunter, placing Talon the Scout will help Irelia jump to the backline and slash through the fragile champions. At that point, the target hit by Irelia will have their energy for their next skill increased by 40%, and Irelia’s skill will also target the champion she is attacking and the highest energy targets on the enemy team, making it quite convenient for countering the Celestial Guardian comp.
Shaco + Thresh

In the current meta, besides Syndra or Jarvan, Shaco is also a name that’s making waves in patch 10.14. Using the strength of the Scout, Shaco can focus damage directly onto the enemy’s backline carry (where Syndra usually stands). In this comp, each teammate that falls will grant Shaco a significant amount of damage thanks to the Dark Star passive. Moreover, Thresh’s ability to pull additional champions onto the board maximizes the strength of this comp, pulling “victims” down for Syndra to eliminate, increasing damage for Shaco, while also boosting Shaco’s attack speed (thanks to Time Out).