Hi and welcome to my guide to zilean the Chonrokeeper,
you probably noticed that
i foucs on:

CDR(cooldown reduction)
Ap(Ability power)
Mana

Pros:
Can finish off enemies Easily (when they run off with low hp)
Good Health
Great support
Works good with teammates
Get exp very fast
Farm skills

Cons:
Can die in early game
(Working on it :( )
(Help me in comments :) )

I take 9/0/21 masteries
I focus in cdr and mana%hp in the masteries

the Utilitys i take are for cdr and mana mostly

the Offense is for bit ap and cdr as well

i dont use Defense since i dont think zilean needs it much

So in items i take AP mostly with some CDR + Ally help :)

The order is

Amplifying Tome (to start with some ap)>Mejai's Soulstealer (if you kill alot and assist)

Boots of Speed (optional) > Sorcerer's Shoes (bit magic pent would help early game)

Catalyst the Protector (helps in health and mana)>Rod of Ages (bit more hp and mana + AP)

Morello's Evil Tome (AP and Mana Regeneration + CDR)

Needlessly Large Rod (Once you have hand on this baby you kill even more easily also helps buying Deathcap)>Rabadon's Deathcap (Domination Machine +30% ap and +155 AP)

Will of the Ancients (Ally help+ap)

In skills you will foucs 1st
on your death machine skill (TIME BOMB)
why?:Finishing you enemies never was much easier

Rewind:Helps your allys :)

Time Warp:you hate running to catch enemy to kill him? SLOW HIM DOWN (also you have time bomb that helps too) also helps your allies catch up him

Chrono Shift:Mark your self or ally to make the enemy damage to heal you/ally!