This is my first build guide(if I can figure this all out!) Please feel free to add constructive criticism to my guide as you see fit.

The runes are self explanatory. The seals and glyphs are blatantly obvious. However, the marks and quins, I find the mix of flat health and scaling health help early and late game. More flats because early you don't have much help from items. And the quints are amazing late game.

Well, here is where it gets tricky. In the defense tree, you have to sacrifice some armor and magic resist to help push your hp and overall survivability. Some of the options I had to leave out(like maxing Hardiness and Resistance) sucked, but overall this is the best skill tree I've found. Feel free to try out some other options if you don't like what I did, after all this is just a guide. I wouldn't recommend you follow it to the letter, everyone plays differently.

So, why rush warmogs? Simple, with Zac you need as much survivability as possible as early as possible. Warmogs gives you a brick of health, and a ton of regeneration to boot. The Liandry/Rylais combo is absolutely amazing. I can't even count how many times I hit someone and walked away then a few seconds later i get a kill. The Spirit visage is amazing for Zac, your healing goes from 4% up to 4.8% I believe. So very well worth the buy considering a lot of your healing comes from the blobletts.

However, if you needed more armor you could sacrifice your Spirit Visage for Thornmail, if you absolutely had to. Likewise if you had to have more magic resist you could sacrifice your Sunfire cape for an Abyssal scepter.

Unstable matter is the main skill you need. From the very beginning. However if you're invading their jungle, it would be a wise choice to take your Stretching Punch, just so you can secure that sacred first blood. That's the only instance that you would take anything other than your W at level one. Your Slingshot has a really long CD, and I don't use it often, so it gets maxed last.

As Zac you need as much survivability as possible, which kind of make a heavy AP role impossible. However as a main tank/bruiser you are crazy good, with your passive you can be super aggressive at all points in the game, except of course when your passive is on CD. Zac can be bulky and hard to escape sometimes, after all your Slingshot has quite a long CD. So if possible don't waste it unless you need it to secure a kill, or to initiate a team fight.

Zac is a HUGE teamwork machine. You have the ability to turret dive to tank a turret while others secure a(or multiple) kill(s). In team fights you have the ability to dive in first to initiate(usually getting focused down first), allowing your teammates to come in and dps down their team(hopefully in time to save you, however if not, your passive should keep you alive through the teamfight.) Your ulti is an amazing disruptor. After slingshotting in, and you Lets Bounce!, you should be able to keep their team on their heels while your team can come in to wreck them. All and all Zac is a really good teamfighting machine!

In summary, Zac is all around a great champion. However he is limited in his possible roles(in my opinion anyway.) I haven't tried him as a jungler, so I can't say if hes good in that area or not. However as a solo top he's my favorite. Zac is a really fun champion to play if you have a good team to back you up. Sometimes you can carry a game, if you win top lane by a HUGE margin and get superfed, however those games are very few and far between. With all of his tools, once you get good with Zac you can be a tide turner in any game. He is good through the entire game, I don't feel like he has a peak point anywhere from the beginning thru the mid all the way to the end of the game.