Threats to Zilean with this build
|Draven||The landing spots of his axes are marked, making bombing him really easy.|
Although most supports should prioritize Sightstone, make sure you get Tear of the Goddess first. This way you start building your mana pool earlier, which will go pretty fast when you are spamming bombs and using Time Warp every time you go back to base and in between.
Once we have Tear, we can get sightstone. Then we complete the Archangel's Staff: this is one of the highest AP item in the game once completely stacked. Also, since we are getting quite a big mana pool, our Seraph's Embrace shield gets pretty big. We can also go for CDR boots early, allowing us to spam bombs even harder.
After those core items, we just keep building AP for max damage and revive health.
Start with Q Time Bomb for harass. Although tempting, you don't want to pick W Rewind second for double bombing, because that will burn through your mana way too fast. Be patient, and take Q-E-Q-W. This will get your harass damage up, which will make it hard for the enemy to keep up their CS.
Time Warp - slow down enemies to a crawl, makes aiming bombs easier
Time Bomb - bomb #1
Rewind - reset Q and E
Time Bomb - bomb #2 STUN
Rewind - slow down enemy or speed up ally, depending on situation
Chronoshift - Don't forget to communicate with your team on how you intend to use it. Also, ping ability ready (Alt-R) when you see opportunities for them to dive, so they know you got their backs.