Hello friends, i play Zilean as supp since S7. I choose to make this guide because i saw that we can't find a reliable guide for Zilean as support. I have more then 300k points on Zilean played as support, so i think you can try this guide.

So, our combos are Q + W + Q + E for stun and slow, E + Q + W + Q for slow and stun.

What you must do is pretty clear and simple: try to poke a lot with your Q, and after you build tear of the goddess you can poke with as more stuns as you can.
As a good support you should always choose your death instead of your ADC's death, so a very important thing is to use your ulty as much as possible to save a team mate.

If you are in a 2v2 fight you should do something like : E+Q+W+Q on one of them + AA on him, and, very important, as you are a mage support you should always stay behind the fight and not in front of it. As you will have a max CDR, you will have another stun in 6-7 sec, so your stun is more important then to take damage, because you have your ulty which is ready for your ADC. Your ADC should be confident that you will save him, and he must play the initiator of the fight. I know that this is pretty hard for an ADC, but this is how an ADC should play with zilean as his SUPP.


Tips:

Every Zilean player should know to use Alt + E + W + Alt + E is his escape and
Alt + R is his save on a fight. Please, use your ulty on you only if you consider that is more important to save yourself then a team mate.

Exemple: if you are a bounty and your team mate which is in a 2v2 teamfight with you have 0 - 8 , well, yeah, i personally choose to save myself because they will probably get a low amount of gold from him, but if they kill you, they will get the bounty amount. In the same way, if you have a lots of deaths and a team mate that is probably to die will give to the opponent team more gold for his death, you should save him.

Thank you for reading this guide. If you have any question, please feel free to comment and ask me.