I have been playing League of Legends since 2018 with short break in (2018-2019). My peak rank is Gold II (which isn't really flattering), but I believe I could achieve at least Dia IV with my experience.

I always used this website to get runes and some tips for champs I wanted to play. I think Zac MID (as it is now) is really overpowered and should be abused by many.

This season I have played over 78 games as Zac MID (and will continue playing) with winrate over 56%.
Note: More than half of my lost games I WASN'T "out of the game" (= unable to play, do anything, kill champs).

Generally speaking you CAN'T always win playing Zac MID. But if you got the right conditions (about which I will tell you later) you should definitely play this BEAST.

Before you play this BEAST and carry many games you should make sure to meet these requirements:

1.) Experience with Zac (any role(even JG), engage setup, team fight)
2.) Enemy team (enemy team SHOULD NOT have more than 1 tank and 1 bruiser)
3.) Your team should be balanced (not all AP champs, should have decent crowd control + damage)

Before the game "starts"
If your team has at least 2 more champs with crowd control and enemy team DOESN'T consist of "early champs" then you should try invade. You can engage and your team can follow up or the other way around, most likely when you use your ignite it's a kill. Be careful because your 'E'(jump) have over 20sec cooldown so you will likely ONLY USE IT ONCE.

Early Game
Depends on the match up, but generally you want to get the first 3 caster minions with your 'E' and get level lead, however if your opponent is range harassing you (=ranged) then try to stay inside your minion wave, so they get big damage from your wave if they hit you. If the poke is insane try to exchange short fights and head to your base when you drop your HP. You can afford to lose some farm (don't die farming), get raptors if possible and farm minions under your wave with possible engage on your enemy mid laner: ('E' on him, Q him and hit enemy minion, W) will cause decent damage you might combine that with your ult and ignite (once you got your mythic and half of legendary item, you should be able to clear the wave under your tower with 'E' -> ult -> 'W' and Q opponent with remaining (likely low hp) minions and possibly 1-2 turret shot(s).
Note: You want to land your 'E' on your opponent (UNDER YOUR TOWER) if possible.

If enemy mid is melee, you should 'Q' -> 'W' or 'E' -> 'Q' -> 'W' when possible. With that and your electrocute will cause decent damage.
Note: If you are unsure of your skills, play safe and save your flash for chance of escaping their follow up.

If you are losing (or more frequently not able to "last hit" enemy mid) then it's the best time for roaming. Make sure to buy control wards whenever it's possible. It saves your time (know they don't have bush on you, while you wait for the right time to enage) and gives vision to your team.

Mid Game
At this point you should have boots, mythic and legendary item with few "legendary parts". You should have enough damage to one shot "paper" champions like ADCs and deal decent damage to bruisers like Jax. Wait for the right moment to jump on enemy champions w/o being noticed beforehand.

Late Game
If you are unable to get kills whenever you got your ult ready, then just spend some time power farming to finish your build instead. You should be careful to not let your team team fight WITHOUT YOU. YOU CAN MAKE A DIFFERENCE IN TEAM FIGHTS.
Zac is great champion to engage to dragon/baron pit and trigger dragon/baron to help you kill almost any enemy (not totally fed, nor tank with decent score).

I AM PLANNING ON FINISHING THIS GUIDE SOON!

POKE #1 : E -> W -> aa
POKE #2a : Q -> E -> W
POKE #2b : E -> Q -> W
ALL IN : E (+ignite) -> Q* -> W -> R -> W (spam + flash if needed) -> Q (when available)

*if possible Q champion then aa targetable (they will likely try to flash/escape after E)