As someone who mained Zac for a large part of Season 3 (ranked stats can be found here if you click on season 3 and order by wins/performance), there's a lot in this guide that I disagree with:

• Runes.
  • Greater Mark of Health is one of the most inefficient marks in the game and therefore a very weak choice. Magic penetration or hybrid penetration should be the standard choice for Zac imo, as he doesn't need any tankiness in marks when using armor seals, magic resistance glyphs, defense masteries and Doran's Shield start already.
  • Quints should all three be the same: magic penetration to abuse Zac's base damages or armor / magic resistance if necessary in a specific matchup. AD quints for easier last hitting in lane is a decent option for newer players, too.
• Items.
  • There's no reason to ever take Doran's Ring over Doran's Shield as Zac doesn't have mana and shield provides more health + health regen and the passive. Ability Power is a decent stat on Zac, but shouldn't be necessary that early either when using the correct runes. Opting to be more aggressive vs certain champions (as you describe in the guide) should result in a difference in playstyle, not starting items.
  • Rylai's Crystal Scepter is decent in terms of the stats it provides, but a lot of the gold value is spent on the passive, which - given the fact that Zac already has so much CC in his kit - is not necessary on Zac. Liandry's Torment and Abyssal Mask are stronger (semi-)offensive items for Zac. It should go without saying that these items are better than Rod of Ages, too.
    In general, your offensive items seem to focus on AP more than magic penetration, which is weird considering that the base damage from Zac's abilities is very high and magic penetration is a more efficient way of exploiting that fact.
  • Getting two Warmog's Armors over something like Warmogs + Randuin's is never worth it as a mix of health and resistances will always be a more efficient way of reducing incoming damage.
• Skill Sequence
  • Your first ability should always be maxed by level 9, the second by level 13.
  • That first ability, in this case, should not be Unstable Matter, as Stretching Strikes provides more reliable poke which is stronger in lane. If your enemy is near his minions, it's pretty easy to use it to harass and farm at the same time, too. Of course, Unstable Matter can still be used to farm in case the damage of Zac's auto attack on a minion isn't quite enough, but to max it just for this reason is unnecessary if you learn how to farm properly on Zac (takes some practice).
  • Elastic Slingshot should be maxed second for the increased range & damage + lower cooldown and can be used offensively in lane (from one of the lane brushes, generally) or to initiate teamfights with.
  • Obviously, this leaves Unstable Matter to be maxed last since the %HP damage is more relevant in the late game as enemies gain more health.
• Masteries
  • I don't mind going for a lot of points in the Defense tree, but 30 is a bit much imo. Personally, I use 4/26/0 on Zac, taking 4 points in Sorcery in the Offense tree while not taking Enchanted Armor (not as great as it sounds), Swiftness (can be taken situationally, but shouldn't generally be necessary with proper use of Elastic Slingshot) and Runic Blessing (weak considering it only regenerates after respawning, not recalling) in the Defense tree.

Because of all that + the lack of depth and BBCode in the guide, I have decided to downvote for now. However, I'd be willing to look at it again and reconsider should you make any changes, so feel free to comment here or PM me if you do.

Good luck! :)

-C2V
-BBCoding

Do it and ill check your guide further. You need C2V in order to stop random downvotes (like turbofean, he didnt even explained why he did it) and BBCoding or NO ONE will read your guide.

Other stuff:
- 2 Warmogs arent viable
- RoA isnt a very good choice, It does gives AP and Health, but also MANA (wich is useless on Zac. For that stats you should get Rylais.
- Doran Ring start is useless. Zac is manaless.
- 30 Defense?
- Dem Runes

I usually dont give downvotes, but this guide may deserve one. You need a MAJOR REWORK. Fix all the stuff I just said and it might turn into a good guide. Also fix the Walls of Text, I want that on books, not on a guide for a game.

Good Luck

Have you tried the build yet??? did not think so!

-1

Situational items are nice. i agree that AP and Health scales on Zac . Rod of Ages is mainly for the AP and health.. i love this build.... recommend using it!

Why would Rod of Ages or Doran's Ring ever Work on Zac? I agree with what has been said about Runes and Masteries too.

well if it doesn't fit your likings then move on. Every has things that work for them and I'm sharing one that works great for me. :)

Pls my eyes...runes are way too bad(quints and marks)
masteries too. 30 points in defense is way too much.
Why would you build Rod of Ages on a manaless champ?
There is not a single bbcode in your guide.