is an extremely unhealthy champion. His ult is an almost full health revive on a 60 second cooldown without any cdr. If a strong adc gets fed like say, i dunno, 
, the game is pretty much over, cause even if your team can dive the draven he will just come back up for free and demolish your team. His ult should be on a taliyah or vi ult cooldown at least.
you dont love zilean having ult up for two teamfights late game? xd
Umm.... ok, I was a Zilean main for a while, hes still one of my favorite picks. I stopped playing him because of his inability to carry. Now, granted Im gold 5, so Im sure hes better the higher elo you are, but no, just no. He technically has 3 spells, Q-E-ULT. His W is the CD reducer of his Q and E.
IF you want to know how to play against Zilean, just don't kill the person he ults, once the timer is off, then kill said person. There, that's it, easy money.
Zilean does have incredible scaling with his Q, HOWEVER, its a heck of a hard skill shot, and to get the CC on it, you need to QWQ and hit TWO skill shots to get the stun.
Zilean is great if your playing with a friend/person who is a good player you trust, but if your on your own in solo q, then no, zileans not going to snowball and go crazy. He is, however, a safe laner, that if the team goes even after laning phase can do extremely well and carry from THAT position.
Edit: Just looked you up, you obviously are better at the game than I, and I see your last game Zilean must have caused you some problems, but take a step back and think about it. Hes rarely played, and there are about 15 other champs that actually REALLY need fixed/nerfed before Zilean would even be considered op, don't you think?
His ultimate is fine. It has a clear indicator on it, so if he uses it on someone you can ignore them until it wears off, you can tunnel Zilean and force him to have to use it on himself or use it early and die. You can simply just pressure past the ultimate, just dealing as much damage as possible and setting up to insta-gib the carry the moment they get back up.
It has quite a bit of counterplay and is an incredibly well designed ability. Stop being salty.
You lose one fight versus him and you come here claiming for nerfs.
Childish. Zilean doesnt need any nerf.
