Pros:
Great sustain early and throughout game
The new jungle is easy for him: fast and safe clear
Awesome ganks at all stages of the game (after level 3)
Engage is best in the game
Great at tower diving
Late game monster tank
Passive is almost a get out of jail free card, and used wisely is the best kind of bait
Surprisingly good sustained dps thanks to cinderhulk, spamming W and % health damage
Can make insane plays because of slingshot.
Great at chasing/running away
Dragon/Baron steals are so good: make sure you have those warded


Cons:
Can be counter jungled: especially if you are lacking awareness and adequate wards
Doesn’t do lots of burst damage
If your team doesn’t have a lot of damage you will most likely lose
Very team reliant in terms of ganks, etc.
Not a true “carry” style champion
Can’t push towers well
Mediocre minion wave clear early

TL;DR

Max R>E>W>Q

Core Items Stalker's Blade: Cinder Hulk, Spirit Visage, Randuin's Omen, and Ninja Tabi.

Focus on ganking early, but never go in if your laner doesn't have the damage to get a kill or force a flash. You can't really 1v1 people early unless they are low until late in the game. Teamfighting you hop in, ult, and spam W and Q. Find interesting gank paths to gank more effectively thanks to slingshot.

Build Zac as a tank, but before picking Zac make sure your team has sufficient damage. The only time Zac is useless is if he jumps in and no one can take advantage of all of his disruptive power.

Early:

1. Spamming your abilities is necessary
2. Figure out the rhythm of his blobs: auto attack consistently but make sure to never let a blob go to waste in a jungle camp, its like hurting yourself on purpose.
3. Before going back make sure you’ve ganked at least two or three times unless your lanes are pushed, then farm as hard as you can.
4. Get the scuttle crabs: free gold/xp and for Zac especially FREE HEALTH. Definitely don’t risk a fight alone for one of these, however.
5. If counter jungled make sure you buy 2-3 wards every back and ping for help from your mid/top/bot lanes if you see a threat. You can’t fight full health enemies alone this early.


Trick! Often to snowball midlane I will grab one buff and two camps so I get level three as fast as possible so I can gank mid with elastic slingshot. If I’m feeling low on health I go do scuttle crab right after wolves to get my health back up thanks to my blobs.

To get a kill, you need your mid to be aware of your gank and they have to do the majority of the damage.
Then set up in an unwarded bush. (It may be a bit tricky to go straight from the bush onto the enemy because slingshots range is still low at level three, but if you can land it you have a great advantage in the gank.)
The key here is to land your slingshot directly behind the enemy mid laner so they are pushed towards your mid. There are several dangers here:

1. If the bush is warded, your gank probably won’t come off.
2. You and your mid might not have enough damage to get a kill or flash
3. I’ve often had my the mid laner die because they committed to hard and you end up going 1 for 1 which sets them back.
4. It’s not easy for you to die in this gank, so don’t worry too much about that but if you’ve cleared poorly and don’t have much health for the gank you might give up your passive.

Mid game:

This part of the game is all about objectives. You should, whenever you gank bot lane, consider going dragon. That’s why I recommend only ganking top once or twice in the early to mid game so that you maintain vision and control over the dragon pit. This includes the scuttle crab which is vital since you often don’t get a chance to back and buy wards and you should be buying a sweeper lens after your first back.

1. Zac is great at ganking, don’t give up. If you gank and just force the enemy back, pretend to back out, hide in the bush and jump in again. It will often surprise them if it is unwarded and you can do that only at the expense of time and experience since your abilities don’t use mana or energy or anything finite and you can always regain health from your jungle.
2. Pretend your jungle is your personal health dispensary. To be honest, for the most part you are not going to be spending your days farming like a Yi or Shyvana. Your goal is to gank and snowball your lanes as well as provide objective control. That means your go to your jungle to get your buffs/give your buffs to your laners or to get a little bit of xp/gold and most importantly health. It’s very hard to die to jungle monsters if you are juggling your blobs and abilities well, especially after finishing spirit visage.
3. Dragon/Towers are paramount. You want to know when a team gains an advantage? When a tower or dragon is taken. Not only do towers give global gold, but they grant vision, allow lanes to push, and make it hard for the other team to push your lanes. Also dragon buffs are very important, and five dragons is a game changer in a number of ways.

Late game:

Late game is all about teamfighting. Every time you have an opportunity to pop into the enemy team (as long as you have back up) you should take it unless your team is way behind.
1. Your engage is legendary and your Ultimate should be used to disrupt their team into chaos.
2. Spam W during your ult for maximum damage. Collect blobs, but don’t go way out of your way to do so.
3. Your E does the most burst damage since you’ve maxed it, so don’t be afraid to use it in short range after your initial engage to kill enemies. If they are close, just hit it twice to use it as quickly as possible.
4. Be fearless: you are Zac, the terror of gelatinous fury and should never be scared to engage.

Item builds should always be team composition dependent. But I've found that some work better than others for different reasons, and that for Zac, you have a core of items you should always buy.

1. Start Machete and pots with a trinket ward.

2. First back: Make sure you have enough gold to buy the upgraded skirmishers blade, ruby crystal, boots, and a sweeping lens. This costs about 1400 gold to include a health pot or two and a green ward.

3. Then you want to finish your cinderhulk enchanted jungle item as fast as possible.

4. Your next buy should be Kindlegarden.

5. Next you want to start to figure out what to build based on the other teams composition and who is fed. That means either finishing Spirit Visage as fast as possible or getting a wardens mail before spirit visage. EITHER WAY YOU NEED SPIRIT VISAGE FOR THE HEAL AMPLIFICATION: SOONER IS BETTER.

6. Next you want to complete Randuins Omen for the health and armour.

7. Now your build is very flexible. Warmogs, Zz'rot portal, Ohmwrecker, Righteous Glory, Frozen Heart are all very viable choices. I like Zz'rot portal a lot right now, and Ohmwrecker is very strong since Zac tower dives frequently. Only recommend tank items that give health unless you KNOW you need to build insane amounts of resistances because the team is all AD/AP or someone is insanely fed like an ADC or an AP carry.