I will admit, laning against sylas does suck. He spams Q and his passive to perma shove the lane but he has many weaknesses. A good start is the fact he is melee and so he gets poked very easily. His burst comes from weaving auto attacks into his spells early in the game but if you position correctly he cant really get onto you without using flash e. He also cant use his heal on non champs and his mana costs are very high once he starts spamming spells.
A good tip is to also play champs against him that have weaker ults or ults that he cant abuse. Champs like ryze, jayce or kass...etc. And most of the time, the ults he gets dont synergize well with his kit as much as they synerygize with the champion that owns that ultimate.
Also a good point to mention is how long his spell rotations are fairly long early in the game.
The last tip is grouping is an amazing counter to him. He suffers from the same problem of any back line diver/disruptor/ap bruiser similar to Diana where unless he is super fed and has a fair amount of AP he can be bursted very easily.
