To make it hard for Sylas to land his E on you, always make sure you’re behind at least 1 minion so he cannot land the CC on you.
If Sylas is able to get on to you, try to disengage as quickly as you can. A lot of Sylas’ damage is dealt when he is on top of you. Try and stay at max range as much as possible to make it harder for him to get on to you.
Sylas has really good map pressure and can roam with ease. If he has left lane and started roaming, communicate with your team and push him in to deny him gold and XP.
Always keep an eye on which Ultimate Sylas has. If his Ultimate R is a very strong team fighting Ultimate, try to wait for it to go on cooldown before forcing a fight.
Look out for flanks as Sylas will often try to flank in a team fight with his E to get onto carries.
Make sure no one on your team is stood too far forward or out of position as it may allow Sylas to pick them off his with his E and start the team fight.
Sylas’ level 6 power spike is really strong depending on the matchup he is in. If your Ultimate is powerful, then his Ultimate R will be powerful too.
The longer the game goes the stronger Sylas will become. Avoid feeding him in the early game to reduce his strength in the mid and late game.
After Sylas has completed his first item, he will have more lane pressure and will look to play offensive. Up until that point, try to abuse him as much as possible.