Sup guys. I'm RAWDEAL, a mere Gold player on Oceania. I'm writing this guide for a friend who really enjoys playing Zilean. Since she never found a guide she personally liked, I decided to write this for her and anyone else who's looking for advice on playing Zilean. This is my first guide here, so bear with me.

I'm interested in unpopular picks and watch streams of them all in Diamond/Challenger games to see how those champions are played, and seeing Zileans with this build actually seem to have a big impact lategame. This isn't a long guide and I have no real credibility but I can guarantee this is an effective and really fun way to play Zilean.

Zilean is kinda strong in low ELO laning. No one really sees him, and people can be a little overwhelmed by the damage of his bombs at first. Make use of your strong poke and auto attack range to whittle down the other laners, eventually forcing them out of lane or into an easy kill.

There are some supports to watch out for, like supports with hard engages. In cases like this, try to poke from safe areas (behind minions vs Blitz) and survive the lane. If they hard engage you after 6, your ulti will be really useful.

Poke with bombs and auto attacks, don't over extend and beware of your own squishiness.

This is where skirmishes, towerdives and fights over objectives start to happen a lot more. Sadly you have nothing to really offer in an all-in circumstance apart from your Guardian Angel ulti, but in other situations you've some great uses. When catching out an enemy, Zileans slow/speedups to your team will likely net a kill, it's a strong ability with a long duration. His ultimate is actually amazing when getting towerdived or when used on your diving assassins, and his burst damage is decent enough to make a difference.
You'll want to stick with your ADC and keep them safe/net kills with them. As long as you're both positioning well with sight, no one will be able to 1v2, 2v2 or chase you both down for kills.

Not many people are used to fighting a lategame Zilean who bought damage rather than support items now that there's more gold available to supports. I've won a lot of my games with my friend playing as Zilean because of the huge & unexpected burst damage. Bomb your teammates that initiate for more backline damage and ult the biggest threats on your team (fed jax, vayne, fizz etc) while slowing enemy divers. Your ADC should be safe at the backline with you and your burst damage against assassins, but your peel isn't the best. Try to keep the map warded and time your ultimate correctly and you'll be an annoying old burst mage by the end of it.

Pros

- Great poke champ, has a good advantage in lane with his AA range

- Speedups and slows for disengaging, makes poking feel safe in most instances

- A free Guardian Angel for anyone on your team at lvl 6

- Ridiculous damage in lategame teamfights

- Fairly strong laner as people don't know how to deal with his poke

- Cool passive. Your laners will usually hit lvl 2 & 6 first, two milestones in the laning phase. Will also help your team get back in the game when behind.

Cons

- Mana intensive champ

- Squishy as hell

- Offers no REAL utility to the team. Don't pick Zilean if you have little CC in your composition or you'll lose teamfights

- Does little damage as well as offering nothing in midgame

- Not a strong support for all-ins, can't protect ADC before 6 very easily

- You won't feel any power 'til lategame unless your enemy laner is a baddy

- Generally not enough damage, CC, gap closers or tankiness to be considered good in the current meta.

Pick Zilean if you want to play a whacky old guy who clocks people out with his bombs.

This was my first guide here so I didn't really do it too well, there's a lot of information in the notes themselves but I'll leave it like that for simplicity.

Disclaimer, I'm a terrible gold player and made this for a friend so she could play Zilean better than she did.
(May add more to this soon about matchups & such)