Against sylas, Sylas doesn't have the strongest of clears in the early game. You could use this to your advantage by invading him and trying to set him behind.

Once Sylas hits level 6, he can steal somebodies Ultimate with his Ultimate R. Depending on the Ultimate he steals, his strength at this time will differ.

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. Against sylas, Sylas does not have the strongest of the whites in early play. You could use it to your advantage by invading it and trying to put it behind. Once Sylas hits level 6, he can fly some Ultimate bodies with his Ultimate R. According to the Ultimate he flies, his strength at that time will be different. The stronger Sylas will become. Avoid feeding him in the early game to reduce his strength in the middle and the end of the game.