The mentality I brought to Zilean here was a Support caster who can feed and protect an ADC till late game, then turn into an end-game ap carry or nuker as well as support when a dedicated support would start to become a little less effective.

Quick cooldowns and huge AP will give you 2-3 600-800 damage bombs in a second, leaving your prey limping away with an execution bomb strapped to him.

Almost constant speed buffs will keep you and your team away from sticky situations or initiating without hesitation, as you resurrect and provide escapes for your team constantly.

Before 6
- Cast Q Time Bombs only on the enemy heroes & avoid taking Minion Kills
- After point 4 remember to speed boost your ADC in and out of clutches, or to escape yourself

Late-Game
Spam your W constantly if your ult is on cooldown. Generally you can rewind every 3 or so seconds, making your ult cooldown stupidly short.
Attach bombs to anyone who comes into range. No exceptions.
Attach more bombs to anyone who comes into range. No exceptions.

Run from everyone. Every time. You often can't trust people to intercept for you. If they do, keep your ult handy.

I build coin, then take no minion last hits at all unless my ADC is not in the lane and even then I'll refrain from pushing the lane.

You're out for the duration of your Mana (using first your Clarity then your Mana pot) trying to keep their heroes away from the ADC. Early game it's better to do slightly more damage with your Q (Time Bomb) than to have a shortest Cooldown on your W (Rewind), so you want to treat Q (Time Bomb) as your priority.

Get Tear/Chalice to give you the Mana to cast continuously, build Cooldown Reduction (Chalice)

The core build will have you casting your rewind every 3-4 seconds your ult isn't ready. You should have enough Mana with Tear, Chalice and later WoTA. This should allow you to rez very frequently & harass constantly.

Keep an ADC or disabler nearby at all times as you're low on hp and escaping once they are in your face is not your strong suit.

don't be afraid to tank a tower if you're low & want your ult to rez you with more life. Just remember to die out of range of said tower when you die.

Spell Vamp - Keeps you healed, turning your spammy timed bombs into constant healing.

Cooldown Reduction - Keeps them scared of you. If you cast with slow cooldowns they will rush and kill you between your bombs. Build cooldown reduction to keep your damage up, but below 6 keep your Q (Time Bomb) higher level than your W (Rewind), as you can't kill them quickly enough even if you're spamming them.

I use cooldown reduction Glyphs, mana regen seals, mnagic pen marks & quintessences.
3.7 Mana regen Per Sec
-7.5% Cooldown Reduction
+14 magic Penetration