Most people think of Zac as a jungler or a solo-top bruiser, but what they don't recognize is Zac's incredible ability to soak up damage. With health buffs and regeneration, Zac becomes a late game juggernaut who can win teamfights just by being around. Early game, his recovery power is unmatched, as taking out minions essentially earns him health, just be careful not to overextend. Watch out for this guy stomping around bottom lane, and definitely do not feed him.

Pros:

Can take a lot of hits
Slows to assist team mates
Slingshot works to avoid death
Slingshot also finishes off runners
Amazing farmer
Awesome in teamfights
Passive is amazing

Cons:

Tank build doesn't deal a lot of damage
Needs support to work well
Abilities can put you out of position
Early game wimp
Lots of trips home

Cell Division:
This makes Zac the powerhouse tank he is. Gain back health from farming without the items needed and defy death in team fights. Early game especially, this allows Zac to turret dive easily, but just be careful to keep at least one of the blobs out of firing range.

Stretching Strike:
Use this to slow enemies fleeing from your aggro partner or ADC. Also can be used to trap enemies near turrets. Level this up first, but max after unstable matter.

Unstable Matter:
Your primary farming tool. Use this in teamfights and on minions to plow through and gain back health. The cooldown on the ability is measly, so feel free to spam it whenever you like, just keep an eye on your health.

Elastic Slingshot:
This ability is awesome, but to be leveled last. Use it to escape a lost fight, or to save another one from the jungle. This is Zac's way of entering and leaving fights, as well as finishing off ADCs who run away. The knock up effect is invaluable, but the cooldown is long, so use sparingly.

Let's Bounce:
Your answer to team fights. Disperse enemies, regain health, and finish off weaklings while in this mode, but make sure to wait until your team is already nearby or there. The cooldown is long, so wait until you know the move will have the most effect. If necessary, Let's Bounce can be used as an emergency escape option.

Zac will need to work with a functioning team if he is going to be a successful tank. His role is as flexible as his goo, so make sure to change it up as you go. During the three phases, he will change like so:

Early Game:
Use stretching strike to slow enemies for your partner and unstable matter to wipe out minions. Elastic slingshot can be used to ambush bush dwellers and to escape lost causes, but should generally be spared for times when it serves the most purpose.

Mid Game:
Zac is now free to roam, so try an visit mid whenever possible. Use the slingshot to pop in unannounced and slow their escape with stretching strike. Use your unstable matter to save turrets from minions.

Late Game:
With your massive health, feel free to tank as much as possible for your team. Break up team charges with let's bounce and finish off with the slingshot. The stretching strike is good for slowing as usual, but you can now finish off enemies at half health with the moves you have. Turrets can be taken out by Zac rather easily due to his massive size, so use your heft to your advantage and dive.

Zac is a slow, but sturdy tank, so move cautiously, but make sure to be the tank still. Only bash in when you know you'll serve a purpose. I will try to update this as much as possible, but thank you for reading my first guide :)