Zilean is super squishy. Focus him rather than the enemy ADC in lane to increase your chances of getting kills.
Once Zilean is level 6, it will be harder for you to get kills in lane. Whenever his Ultimate R is down, try to abuse him to get kills. One way of killing him when his Ultimate R is up is by locking him down with CC so he is unable to use it on himself.
When looking to trade, try and bait out his Q before initiating a fight. This will make it impossible for him to CC you if the first Q misses.
Zilean will often be left alone to clear waves with his Q. Try to pick him off before sieging an objective to make taking it much easier.
Avoid grouping too closely as it will allow Zilean to land a good stun with his Q and potentially start the team fight.
Playing around Zilean’s Ultimate R is crucial to winning fights. Try to force fights when his Ultimate R is down.
Zilean doesn’t have much damage at level 1 but once he has access to both his Q and W at level 2, his kill pressure will increase. Even though it’s unlikely he will get a level 2 kill, his damage output is nothing to be messed with.
At level 6, Zilean’s kill pressure doesn’t exactly increase, but his survivability does. Whenever he uses his Ultimate , try to abuse the cooldown and play aggressive while it’s down.
Once he has max cooldown reduction, Zilean’s Ultimate will be on a very short cooldown. It will be harder to get picks during the mid and late game as he will save them with his Ultimate .