I'm looking at your match history, and here's some advice from a fellow Zilean player - I'm currently rank 6 with two Mastery 7 tokens.
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You die a lot. You need to have 2 deaths max in a sub-40 game, or 3 in a 40+ game to even compete. Not only is this mandatory S policy, it's also good game policy. Work on this before anything else.
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Take
over the coin line. Zil has huge auto attack range, and bombs make getting gold easy money. Quicker money, better items, yada yada
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I'd avoid
. You want CDR, but not from that. What I'd suggest for a build order is this:
Start -
x3
1st back -
2nd - 
The thing to keep in mind is your first few items should be about gold. Frost fang gives you more, and tear allows you to stay in lane more. Also, the less you're dead the more you make.
My final build tends to look like this:
(this makes his revive provide more health, and gives you speed),

Some consideration can be given to removing Seraph mid-game to pick up
for more speed when you don't need mana, but that's up to you. And shouldn't be applied broadly. A Zilean without mana is some bad bad news.
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Make sure you're maxing slow first and your bomb last. His slow down/speed up ability is the single greatest gank-removal method in the game. I more often use it to slow enemies, but the occasional boost for a kill has occured.
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If you want an S, improve 3 things:
A. Your death count.
B. Your assist count. The active on Queen's and Redemption should help here.
C. Your CS. Banner should help with this assuming you set it up for success:
Start as close to your base as you can (with consideration for the point of the game)
Don't send a promo into their minion push
Always give it to a Siege