HELLO! I am a gold 5 Zilean main from NA with over 350k points on him so far. I think Zilean is an incredibly rewarding champion due to the fact that not many people play him. You can surprise quite a few people with his kit. Take this with a grain of salt as there exists a lot of conditions to winning as a support Zilean. With that being said I hope you learn something from my guide and grow to be a better Zilean player for it.

My winrate:
Season 6
season 7

The build I have presented is pretty much the standard in Zilean support nowadays. Pretty much every pro-player builds this way due to its gold efficiency. Redemptionis the staple first item on a vast array of supports. Buying tear as a support puts you at a disadvantage let alone trying to stack it. Zilean stacks it moderately well but that items should be avoided in favor of the meta items. Eye of the Watchers provides you with the wards you need and some ap/cool down reduction for your troubles. Ionian Boots of Lucidity offer you essential cdr. Boots of Swiftness offer you a bit of extra mobility without compromising you if instead you had boots of mobility. Locket of the Iron Solari provides your team with valuable shields that they need. Mikael's Blessing is instrumental in keeping your carry from being locked down. Due to Zilean's poor itemization his final item is weird. Most meta would be Zeke's Harbinger, but that doesn't really make sense as a last item. Second most would be Rabadon's Deathcap which is nice but in terms of the way we're building isn't really too sound either. Thanks to the guardian angel changes your last item is going to be matchup specific from now on. The new magic resist items aren't a bad shout, spirit visage increases the heal from your ult (if you happen to ult yourself). Adaptive helm seems really nice if you get caught out by their midlaner or ap jungler. Overall the build is still pretty much the same.

If you are facing a hard counter in laning phase you should simply just tell your adc that it's going to be a bit rough. With their help you should be fine if you properly freeze/farm under turret or ask for jungler assistance. It is important that no matter what you should not get denied a laning phase or be crushed completely in it. You are a scaling champion that excels in the mid to late game. Meaning if you are crushed by a zyra or a lulu that also scale, your personal mid to late game will come far too late to matter. If you feed first blood but your adc trades a kill it's fine. Early dying isn't great but it isn't the end of the world, being denied exp because you didn't communicate with your adc to freeze IS.

Nightmare scenarios aside if you are in an easy or moderately difficult lane, you should help your adc push by bombing the caster wave of minions (careful not to take their cs). Remember to always E and then double bomb and auto to make sure your harass onto your enemy is going to work. If you have a mobile adc who can dish out a lot of damage (Draven, Lucian, etc.) don't hesitate to speed them up for some harass onto the enemy. Your early bombs are 55 damage (very low), make sure you don't trade stupidly even against easier match=ups. Your most noticeable power-spike is your ultimate; Utilize it to bait in enemies to kill your adc and ult them when they are close to dying to turn the fight in your favor.

You should almost always stay in botlane to soak up exp, but if you are crushing it you can roam to other lanes. Mid is the closest and clearest target to said ganking. If the enemy mid is immobile and your roam is pretty early you can blow his flash for free. In doing so you take away a powerful summoner spell from said laner and give the advantage to yours. Helping your jungler secure the enemy buffs is also noteworthy. You can bait the enemy jungler into a 1v2 if you stick to the back and utilize the vision you get out of your wards/ally player.

Always try and prioritize the immobile champions and try to pick them out by slow+double bombing them. You can utilize a double bomb on a minion wave to stun multiple opponents in a short time. This catches out a lot of people who aren't prepared for it. Use your ultimate wisely, sometimes it's a friend who is simply caught out. Other times you need to prioritize the actual carry of your team (maybe a fed riven). This may cause concern with your adc who considers themselves the carry and the person you should ult. Many a time this is simply not the case. You are the only one who can decide how you use your ultimate. Your said usage of it can result in games heroically won and games where you simply throw. Try and practice his kit as much as possible to learn how to get the most out of it.

In notepad or word you can warm up before a game by typing in "eqwq eqwq eqwq eqwq eqwq eqwq". You should repeat it until you are familiar with the combo and can do it quickly without messing it up. If you mess up the combo (happens to some people) it turns out as e+w+q meaning you have no secondary bomb available. Try and practice as much as possible before you bring him into ranked. Good luck! P.S. You can ult allies by pressing R on top of their icon to automatically ult them if they're near enough. This prevents any accidental ulting of undesirables.