[BUILD GUIDE WILL BE FURTHER CORRECTED TO REFLECT MORE INFO AS I COME ACROSS IT]

Welcome to my Zilean build! Let me get this out of the way right now: I know Zilean is always a support character. I agree, he is one of the best supports out there, and as such, it is his first and foremost function (certainly his best). However, the idea of tinkering with champions is what LoL is all about at the end of the day.

This build will focus on Zilean performing as a heavy nuker. Though his damage is focused almost entirely on lane harassment, I see potential in Zilean instead focusing that damage towards dealing damage. Now, good ol' Zil won't be able to do the entire thing by himself (he is, after all, not a very durable champion). Instead, he's a team player, guaranteeing a kill for team fights and ensuring a high-assist game.

Please note that bolded words will have a definition at the end of the chapter for newer players.
support character: a champion whose purpose is to aid other champions on the battlefield.
nuker: a champion whose purpose is to deal large, quick damage to enemies.

Zilean usually focuses on magic penetration and mp5. The hope is that this constant stream of damage via repeated Time Bombs will keep enemy champions back. This build will focus on AP in addition to enhance the damage of Time Bomb itself, and hope to land much harder hits that can cripple or even kill an enemy.

magic penetration: the amount of magic damage that ignores an enemy's magic defense.
mp5: mana per 5 seconds; the amount of mana regenerated in 5 seconds.
AP: ability power, usually affecting abilities (seen in green text in parentheses, in ability descriptions).

Despite this build focusing on Zilean's damage, his squishy magic build will always make his mastery tree more utility than anything. There's a 10/0/20 split; for Offense, there's 1 point in Deadliness to reach the requisite points to invest in Sorcery and eventually Archaic Knowledge (for magic penetration). Archmage's Savvy and Burning Embers are there to boost the AP for heavier nuke damage.

In the Utility depearment, we advance down whatever will give us better Time Bomb/Rewind utilization. Often, Zilean is given Utility Master, as he is the main support, but we're skipping it to go for more damage.

This is the big, controversial part of the Zilean build. Normally, the focus is on a harassing, quick-shot Zil; my Zilean focuses instead on AP for those harder hits. Time Bomb gets .9 damage per AP, so the almost 165 AP at level 18 (largely due to items) is about 150 damage added. With Rewind for a double Time Bomb hit, that's a total of 940 damage, give or take for magic resistance/penetration. Half of it will be instant, and the other half will sneak up on a cocky enemy who thinks he's doing fine health-wise.

Early game Blasting Wand will give your Time Bombs a bit more deadliness and set you up for a Rabadon's Deathcap (suggestion courtesy of Periphetes). First though, I grab Boots of Speed for some getaway power, because Zilean will be a huge target for enemies. Next, I buy Needlessy Large Rod and eventually Rabadon's. Rabadon's allows for lots of AP for a heavy hit. Then pick up Ionian Boots of Lucidity to give you a great cooldown reduction in addition to movement 2, and you're good to work towards Soul Shroud for mana regen.

cooldown: the recharge time on a specific spell or ability.

Early game Zilean is practically the same in this build as many others; Zilean must stay with (mostly behind) ally champions to harass opponents and keep them out of your lane as your partner farms. Stay in range of experience to level (you'll notice leveling gets harder later on due to your inability to gank as easily as most other innately powerful characters), and stay out of range of the enemy.

Don't throw out a Time Bomb every time you have one. It may seem good, but keep in mind that your mana is precious, and will run out very easily. On top of this, most champions won't run from a few Time Bombs. Half the time, they won't even know what hit 'em. Use it when they get too close to your side of the creep fight, or they are low on health. If you see a low-health champion who is in a hard to kill spot (behind the turret, with a friend, etc.), just let it go.

After you learn Rewind, utilize the double Time Bomb. Use the keyboard's Q-W-Q to TRT (Time Bomb/Rewind/Time Bomb). The first Time Bomb will explode instantly when the second one replaces it, and they will be so surprised they won't be able to run from the second and will take massive damage before they realize it.

Just keep in mind that you are helping friends and biding time at these levels. Farm, don't go running around uselessly, and throw a speed-up on your ally occasionally to help them escape attackers.

lane: the path that minions follow.
gank: an attack in which the attackers have a severe advantage, such as more numbers, more health, or more levels, or a combination of those factors.
turret: the towers that deal damage to enemies in each lane and the base.

Now that you've hit mid game, you'll notice you can throw on two Time Bombs for quick and fierce damage, slow them down, lop on Ignite for some DPS, and then proceed to auto attack them until the second bomb explodes for truly surprising damage. If a friend is around to help, the enemy will fall.

You might try your hand at some kills on your own, but chances are you still need a team at this point. You'll find you quite enjoy pushing back enemies at this level so your teammates can crush towers after them. Just be cautious, and remember that forcing them back to base may not be as good as a kill, but it's not too far from it, so let it go if it's too risky. You're very squishy.

It's your time to shine, now. You should have a score looking something like 2/1/10 at this point, and that's good. Your team will appreciate the help and will be glad to stick by you for those nasty ganks you'll cause with your slows and nukes. If you guys get in trouble, you'll speed up out of there. Hell, you can even revive.

Being alone is not advisable, but you should be able to survive 1-on-1's. Avoid a pair or more of champions by yourself, because you will not win. Stick with teammates and you'll pick up some kills of your own, now. The delayed Time Bomb will do that for you quite nicely.

Time Bomb is a delayed attack that will tick away for 4 seconds, and then erupt for damage in a small AoE zone (don't underestimate it's smallness). You don't need to place it on enemies, either; placing the bomb on an ally, even allied champions, will cause the bomb to tick off and do damage to in-range enemies, but do no harm to the ally.

Placing a second Time Bomb on a target who is already under your own Time Bomb will cause the first to explode for damage immediately and the second to take its place. If there is an enemy Zilean who puts a Time Bomb on you or an ally, you can use your own Time Bomb on the targeted ally to negate the first Time Bomb, and still cause an explosion from your own.

Time Bomb's cooldown is less than that restored by Rewind; therefore, Rewind will always instantly refresh your Time Bomb spell. Use them in quick conjunction (Q-W-Q, or Time Bomb/Rewind/Time Bomb) to do a large sum of damage instantly and even more 4 seconds later. Toss in a slow to stop their almost guaranteed escape (if you caught them off guard) and they will be crushed by an ally (or yourself, if they were weak).

Tossing a Time Bomb on an allied minion will keep melee champions away from that creep, which is useful to keep them away for harassment or farming purposes. If they don't heed the warning and stay, then they will receive damage, and likely retreat anyways. Keep this useful info in mind.

Time Bomb has ridiculous range. Get to know the perfect range, and you can practice what I call the bomb-and-run: toss a bomb on them, and run away before they can react. You'll be so far away you're almost guaranteed escape. If they're hiding directly in front of a turret, and you've gotten comfortable with the range of Time Bomb, target the spell on the enemy and as soon as Zilean drops the bomb, run back. With enough practice, you should be able to escape before the turret gets a chance to fire on you.