Hi all! My name is Zach, and this is my guide on how to play ME as a champion.
Zac is primarily a Tank with some pretty powerful abilities that scale solely off AP. He is A LOT of fun to play, since his ganking abilities are, in my opinion, the best in the game. Since he is such a tank as well (my build usually ends up around 4000 healh at level 18) and has such multiple powerful AOEs that he can spam in the middle of a team fight, he is an amazing asset in a team if played properly. Here is what I have found to work best over the last day he's been out (I may have played more league than I would care to admit over this past weekend, but we'll keep quiet about that).
Zac's passive gives him INSANE sustain. I rarely go back to base before level 7 or 8 because of this, usually allowing me to buy one or two of my core items outright and gaining a huge power boost instantly. His cell division ability when he dies also allows him to be played much more aggressively during the early laneing phase than most are used to, causing the enemy team to often think they've killed you (wasting an ignite, flash, or ulti to 'kill' you) only to have you reform at half health and destroy the tiny bit of their health that's left with the ultra-spammable abilities he has. I find first blood is easy to attain for your team by utilizing this passive and getting them to stay a bit too long in an attempt to kill you, allowing teammates to get first blood or you to reform and rejoin the fight.
His cell division passive when he is alive is a lifesaver. Each blob heals Zac for 4% of his maximum health, while the abilities all cost 4% of the current health. This means you always heal for a net gain if your ability hits an enemy and you get the blob, basically giving a beneficial cost to using abilities. I recommend using multiple abilities at once and then grabbing the blobs, allowing maximum healing due to the diminishing cost of abilities when your health is lower. Also, if ignited and low, spam abilities and pick up the blobs to counter the ignite and survive (has saved me more than once).
I find it best to start ganking at level 4 when the slingshot is up. This allows you to jump into a lane, knocking the enemy up from nowhere. Seriously, the range on this thing is HUGE.
Thus, the sequence I use is E > W > Q > W. This does a fair amount of damage on its own, even at low levels (the W is a flat percentage of their maximum health, a rather nice feature on Zac) allowing a gank on a low enemy or a coordinated jump with the lane to give an assured kill.
After level 6, I throw the ulti in there if they seem they're getting away (remember, the E and Q both slow, so if the ulti is down those are useful too). This gives a MASSIVE speed boost, knocks up and slows enemies, and destroys their health bar. During the bouncing of the ulti, it is even possible to recast the W bringing in even more damage, all while spreading out little blobs of yourself to pick up and regain health with if needed. Thus, the sequence turns into E > W > Q > R > W.
Pros / Cons
Multiple Slows and Stuns
Great mid and end game
Passive may be wasted if killed in a bad spot (e.g. under an enemy turret)
Not a ton of damage until geared
Takes a bit of playing to get used to
Not the burstiest AP character out there
Creeping / Jungling
I usually start jungling with wraiths and a smiteless red, pop a health pot or two and take wolves then blue with the smite. This path clears the jungle pretty quickly and gives you back tons of health if the W is used correctly. Remember to pick up the blobs!
Zac is an incredibly fun champion to play and I highly recommend everyone at least trying him if the chance should arise. Concentrate mainly on health and armor during the midgame for survivability after gaining a bit of AP early game and come endgame... well, I guess you'll get to see that for yourself.