Not Updated For Current Season
This build focuses on playing a zilean, that can survive, help allies and still do decent amount of damage.
Zilean's true power lies in spamming spells - once in a dire situation or a big battle, throw bombs all over the place, slow enemy guys or speed yourself up.. spam, spam, spam. Since rewind makes all cooldowns a lot shorter, the deciding factor is usually mana. So if you got mana, use it!
The only thing you don't wanna spam is the ulti. You don't have to hesitate to use it; If you are for instance laning and are low on hp, you can use it to get healed. But always be sure to not waste it before some important event - it has a low cooldown, but it still isn't instant.
Zil's runes focus almost purely on mana regen, since in full-combat mode, zilean consumes about 300 mana per 5s.
Even when Zil is in a lane, extra mana is extremely usefull, since it can be turned instantly into gold by double-bombing clumps of minions.
The insane regeneration you get with your runes allows you to turn into combat-mode, do some ganking / saving / harras and then return to the tower and replenish the mana while still in the lane.
I changed the mana regen marks into spell pen marks, since spell pen helps a lot with bomb damage and the mana regen marks were quite weak. Plus, chalice of harmony provides enough mana anyway.
I start with the meki pendant and 2 health pots. Since you need a lot of mana throughout all game phases, get the chalice as soon as possible, followed by normal boots and tear of the goddess.
Then get boots of lucidity and start working towards the soulshroud, which will give you a bit of survivability. It's generally a good idea to start with the kindlegem.
After, get your archangels for a big load of mana and go for rabbadons. And if the game still doesn't end by then, you gotta figure for your own if you need more survivability or just more AP.
If the enemy focuses you down with multiple stuns and tends to kill you off before you get a chance to get ulti, get a quicksilver sash.
And the most important... health pots, health pots, health pots. They really are for the win. They allow you to stay in lane, allow you to survive ignites (sometimes), usefull against teemo's poison, and the best part is: you can afford them. With tons of mana, you'll be farming as crazy and thus able to spend a lot of cash for the pots.
The main idea is to max bombs, then rewind, with one exception at level 4. You want to take 1 level of time warp (not more) and generally not use it until you really need to. It is great for escapes from ganks, helping allies escape, slowing down enemies, you get the idea.
Also, be very careful to use timewarp for harrasment. It has a long cooldown and the enemy can hunt you down if you don't have it ready for escape.
Flash and Teleport. Good combo for a support-ganker-runner.
Teleport allows you to help you allies quick, farm minions from besieged towers.
Flash is more fun; apart from running away and flashing behind terrain, it has one great use. Almost every Zilean got into a situation where he got killed with his ult on and allies where nowhere near.
If you get killed with your ult and there are a max of 3 champs waiting around you to kill you off, you got a good chance of getting away. Flash before anything else in any direction you think they will expect least (it can be right to their tower), then immediatelly use time warp and run as ****. You should be faster than anyone else, if not, slow him and then speed yourself again. You can put up a few pretty epic chases through the enemy forest with this tactic.
And the best part is, teleport. If you manage to get 4 seconds in a brush and you got it ready, you can teleport to a home tower.
Farming is essential with Zil. You have to farm and you have to farm well. On lower levels, you can hardly get champion kills on your own, so focus on harrasment and farming. It goes like this:
bomb level 1: bombs used for harrasing champs, last hitting used for farming
bomb level 2: double-bombs used for killing all 3 ranged minions. Prerequirement: hit every one of them once with auto-attack
bomb level 3: double bombs for killing all 3 ranged minions OR double bombs for killing a bit wounded meele minions
In higher levels, you pretty much bomb any bunch of creeps, always focusing the double bomb on the weakest piece, so that both bombs would detonate at once.
Zil is best paired with taunters / stunners. Best is probably pantheon. Panth stuns enemy, zil double bomb him, then slows, giving panth a chance to deal more damage with auto-attack.
On higher levels in team battles, you basically want to slow down the focused enemies, bomb the weak ones, ult your damage dealers, haste allies running for their lives. Its all pretty obvious.
Just a note on focusing here; In huge battles where you don't know who is where, just bomb anyone in range. It's way better than to just stand there and look silly trying to find the squishy. Don't worry about the focus that much, bombs have splash.
In short terms, this Zilean is hard to catch, hard to kill, spams spells like crazy when in battle and farms insane amounts of minions when in lane. Just be carefull in laning phase, otherwise he tends to die. A lot.
Chalice of harmony
I added this item after playing zilean a bit more. Why?
1) It gives a LOT of mana regen (especially along with mana regen runes)
2) The passive ensures you, that no matter how much you spam spells, you will almost always have enough mana for your ulti.
3) Magic resistance. It looks weak, but: Most of the stuns that hurt are magic. Plus zilean is very squishy before he gets his soulshroud. This means, that the slight reduction in damage you get can save your life long enough to cast ulti or run away.
I was experimenting with spell vamp and it just didn't work for me.
For bombs, spell vamp value is calculated from the bomb damage - to one target. So no matter how many dudes you bomb, you still get only the same X hp from it. And since zil, the way i play him, doesn't have that much of a damage, i skip spell vamp completely.
Zil has two great "unique" things:
1) His ulti, when provided a lot of mana for rewind, has technically 26s cooldown. (provided max cooldown reduction). Meaning that over longer team battles, you can revive twice.
2) A little trick with a weak minion and a double-bomb. If you plant a double bomb on a minion that would die right after the first bomb blows up, both bombs blow up instantly. This can be used for harrasment through farming. If the enemy champions are just out of range, bomb the minions they are hiding behind. If you manage to double bomb a weak minion, the hiding champ will get damage from both bombs.
2.5) Double bomb. Since i use this term all over the place i will clarify what it means (it seem quite obvious but to be sure): You plant a bomb. Rewind. Plant another bomb on the same target right away. Efect: The first bomb blows, the second ticks. Instant AoE damage.