After triggering an enemy Zac'sCell Division, place one bomb on one bloblet and another bomb on a different bloblet. As the timer ticks down, the bombs will get crawl closer together and more likely finish him off. However, if you quickly double bomb one bloblet before the rest get close, only one bloblet will be killed, and Zac will revive.
Placing a Time Bomb on an enemy as they teleport may also get you a kill as they will most likely be teleporting to a fight.
You can combine the use of Time Bomb and Rewind to place two Time Bombs on a target quickly. As this causes the first bomb to instantly explode, this is Zilean main damage combo. If the first bomb is enough to kill the target, both bombs will explode. This can be very effective for farming and pushing. This combo costs a lot of mana, so you may not want to use it for harassing.
This strategy can be a good way to damage enemy champions standing near the target before they have a chance to shield themselves from damage.
Time Bombs grant vision to allies. They can be used to scout unchecked bushes or over walls.
Lobbing bombs into bushes can surprise foes for an attack.
Using bombs on your own or enemy minions, is a good way of zoning your opponent. If the enemy champion is melee, they have to either wait 3 seconds for the bomb to explode, or be damaged by the bomb if they choose to farm that minion.
When sieging an enemy turret, have your allied champions clear the enemy minion wave. Place a bomb on the 2nd and 3rd melee minion in your ally minion column. They will carry the bombs forwards for you, possibly harassing a careless opponent under their tower.
When Zilean has enough AP to kill melee minions with a Q > W > Q combo, he can insta-clear all 6 by walking ahead of his minions to the enemy minion line, then lead the melee minions towards the faster minions. Q > W > Q combo on a faster minion will clear the entire wave.
If the enemy has some sort of shield that can mitigate your bomb damage, use Time Bomb on a low health minion they are about to kill (smartcast helps). This transforms Time Bomb from a nuke with a 3-second windup to an instantaneous AoE damage nuke (without the use of Rewind).
If you have Syndra as an ally, you can place a Time Bomb on a minion she grabbed with her Force of Will. When she throws it the minion will explode, dealing heavy AOE damage due to the combined forces of Force of Will and Time Bomb. This is basically a long range AoE skillshot, which is very helpful at sieging and poking.
Placing Time Bomb on a minion and then having an ally use a knockback ability on the minion (such as Headbutt or Buster Shot) into an enemy champion can deal long-range damage or finish of fleeing enemies.
Placing a Time Bomb on certain uncontrollable pets will cause them to follow their masters around. This is great placement for a no-escape harass or instant kill at low health. This works most effectively on Elise because she has little control over her Spiderlings, and on Malzahar who cannot control his Voidlings without a cooldown in his favor and an enemy in range.
Consider enemy champions shield durations or damage reductions when casting time bomb. For example, after Katarina casts Shunpo, she takes reduced damage for a little over a second. Therefore, placing a single bomb on her after Shunpo will detonate 3 seconds later, with no damage reduction from Shunpo. Or, placing a single bomb immediately, then waiting for the Shunpo damage reduction to wear off before the casting the second bomb will also avoid damage reduction from Shunpo.
Another example is Safeguard, which shields Lee Sin and an ally champion for a few seconds. If Zilean casts Time Bomb on Lee Sin directly, he can Safeguard one second before the bomb goes off to mitigate the damage. The safest way to damage him with two bombs is to quickly double bomb a minion that is in the vicinity, assuming that one bomb is sufficient to kill the minion. This would result in the AoE damage to be that of two bombs, since placing one bomb over another detonates the first and since the target dies, the second immediately detonates. However, this is very difficult to pull off and requires you to have high AP.
This same technique can be used to harass them under their own tower without taking tower hits as well.
Time Warp is an effective way to enable allies/yourself to finish off enemies, engage enemies, or escape from a losing battle. By using Rewind, Zilean can slow or boost an enemy or ally twice allowing an easy kill or escape.
Rewind is Zilean's only ability that is insta-cast. Casting another ability then Rewind in a quick sequence may end up the Rewind being cast earlier, hence the ability input next unaffected by Rewind as expected.
Use Chronoshift strategically for maximum effect. It can prolong the life span of yourself or a team member.
You can bait low enemies to your tower by saving your Chronoshift till the last second to bait enemies into committing to kill you or die.
Typically it is better to save Chronoshift for an AD carry or tanky DPS, don't use it to save a support unless you can tell the teamfight is almost over and no one else needs it. If the enemy team focuses you, use Chronoshift on yourself of course. If you save it you might die and not be able to use it at all. The benefit is the damage and CC you have absorbed only to come back to life.
It can be difficult to cast Chronoshift on the ally teammate you want during a team fight due to champions being on top of each other, visual effects of spells, and losing your mouse cursor. Casting the spell on your allies portrait in the upper left corner of the screen will avoid this problem. If Zilean is out of range, he will walk towards them until he is in range and then automatically cast the spell.
In teamfights, Zilean can safely Time Warp the allied ADC to help them kite back and forth, and Chronoshift the allied ADC as a type of peel by remaining behind the allied ADC or to the side of them. Also, Zilean can slow anyone trying to get to the allied ADC as well, however it only works on one opposing champion, so with multiple opposing champions rushing your allied ADC it may be more effective to speed your allied ADC instead.
Zilean's greatest potential is unleashed around mid to late game, where the cooldown of his abilities will be ridiculously low, thanks to Rewind.
Zilean is viable as a support, trading your early game damage spike for utility.
Placing Time Bomb on top of another provides an AoE stun, which can be helpful during skirmishes or teamfights.
Since all of Zilean's abilities are spammable but mana-heavy, you might find yourself quickly running out of mana in the early stages of the game. To aid this, purchase Tear of the Goddess for decent mana regeneration and for early harassment, which can be later upgraded into an Archangel's Staff. This is especially useful if one is also building other items providing mana like Frozen Heart or Rod of Ages. For support builds, this leaves more room for other items, as this is essentialy the only offensive item a support Zilean would need.
By purchasing Guardian Angel and using Chronoshift, Zilean can be a great time consumer to the opposing team, which allows the ally team to know how to approach an ambush while you are reviving.
Though, keep in mind that Guardian Angel passive will always proc after Chronoshift and generally will resurrect you with a much lower amount of health.
Cooldown reduction is doubly effective on Zilean, since in addition to decreasing the cooldowns of his abilities it decreases the cooldown of Rewind allowing it to be used more often to decrease the cooldowns on your abilities even further.
Zilean's early harassment power can be mitigated by sending a tanky or healing champion to lane against him. You are more likely to win a battle of attrition due to his high early mana costs. Champions with the ability to silence also have an upper hand on Zilean due to his low defense stats
When caught by Time Bomb, you can force him to push the lane by intentionally damaging your minions with the explosion.
Remember to stay away from your allies when he uses his Time Bomb on you so you won't unintentionally damage them.
Ideally you should attempt to take Zilean down quickly with enough burst power to prevent him from making use of Chronoshift. If this proves impossible, consider that you may need to focus on which of Zilean's allies you can kill the fastest over which one is dealing the most damage.
Zilean's mobility with Time Warp makes it difficult to finish him off without a well-orchestrated ambush, for he can easily bait and win a fight with Chronoshift. However, his low natural defenses make him a good target for assassins.
Zilean cannot usually afford to increase his ability power by late game and his utility is less dependent on his item build. Attempting to kill Zilean during the laning phase should be a very low priority compared to farming minions.
Chronoshift is less useful versus teams with high sustained damage combined with crowd control like Kassadin and Jax.
Zilean is heavily susceptible to silence effects. If you have enough silence in team, you can try to silence Zilean when he is about to cast Chronoshift on one of his allies or kill him before he can use it on himself.