You want to play Zac offensively, but only when someone can help you out if something goes wrong. You play Zac mainly using his Q and W abilities. Then using E to escape, or get the final blow. He is a champion that can do plenty of damage, and one which can withstand quite a lot. Zac's passive is very effective and can help a lot with minions. An Unstable Matter (W) to a Stretching Strike (Q) can take out a wave very quickly.
Zac's runes mainly focus on Health, Health ReGen and Ability Power. They increase his early game stats and his scaling runes help him later on in the game.
Zac's masteries focus on health and ability power. You build zac in this way as his passive resolves around health and his abilities revolve around ability power.
Once again focusing on Ability Power and Health
His Q and W are his best abilities which are focused throughout the build. His W is used mainly for healing and Q for damage.
You use flash for quick escapes along with your E and you use Smite for jungling.
Great character that is deadly when fed.
Zac is very good at doing lots of damage in a short amount of time. Pop a W then a Q and follow up with an e to get you back to the fight or if you are low on health, get you out of the fight. Don't hold back on using your ult when you running away, its mainly used for crowd control but is very effective to use when escaping.