Against zilean, 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 t  is down, try to abuse him to get kills. One way of killing him when his Ultimate t 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.  Against zilean, Zilean is super squishy. Focus him rather than the ADC enemy in the way to increase your chances of getting killed. Once Zilean is level 6, it will be harder for you to get killed in the way. Every time his Ultimate t is down, try to abuse him to get killed. One way to kill him when his Ultimate t is up is to lock him up with CC so he is unable to use it on himself.