Darius

Hard Counter to Zac Top. His cooldowns are faster than yours and his passive bleeds you out. Zac's trades take a long time during which he heals back up. Darius' passive makes it so that you can still heal, but enough stacks and you wouldn't be able to heal enough. I recommend going Tank for Darius and bringing flash Ignite. Ban him if you don't have any problem with other laners.

Sett

Sett is a very annoying matchup. If you are starting out on Zac top and don't know how to do his combo's quickly, he can cancel your Q with his E and quickly decimate your HP. His pull can also counter your slingshot, when timed correctly. Sett's W isn't too much of a problem, since Zac doesn't do burst damage at the speed of light, it takes a bit of time to build up the damage. I would recommend bringing Ignite and Flash to the top lane, and build Sunfire & Thornmail ASAP.

Kayle

Kayle early game isn't much of a threat at all. She is very squishy and is melee ranged, which makes it perfect for Zac to fight her. Zac can out heal her early game DPS when played properly. Be careful if you feed her though, as her Ultimate can save her from getting killed by you and possibly turn the fight around if she ranges you properly post level 6. I recommend bringing Flash and Ignite, as Ignite can secure your early game kills since Kayle is squishy.

Tryndamere

Tryndamere is quite an annoying matchup if fed, but early game you should be able to win with your airborne. I suggest waiting until level 3 so that you have maximum airborne capacity, although be careful of Trynd's healing ability and his scaling AD as his hp gets lower. Try not to push up too far as his W can slow you down, and since your slingshot has a very long cooldown early game, leaves you vulnerable for ganks. I recommend bringing Teleport and Flash, as Ignite can't really help you when Tryndamere is past level 6, as he can heal with his Q and his Ultimate prevents him from dying, and helps him to escape you.

Illaoi

Illaoi late game is very annoying and hard to play against. Zac doesn't move around at the speed of light, making it so that her tentacle can hit you pretty often, and her ult will be able to one shot you if you don't build properly. Early game you should try to fight her as much as you can to prevent her from getting CS, her first ability is usually Tentacle Smash, which is very easy to dodge. I recommend bringing Ignite and Flash, but be very careful past level 6.

Fiora

Fiora is a very hard play against Zac because with her parry, she can stun and counter you pretty fast. Fiora also moves around very quickly, making it hard to hit your Q on her, and when you do land your Q she can just Riposte again. Her ultimate decimates your hp and heals her so that if you got her at around 10% and she ulted, she would be able to heal all her hp back by hitting all your vitals. I recommend bringing Flash and Ignite, as you may be able to reduce her healing with ignite if used properly. Try to shut her down early game if she doesn't get Riposte first level, you should win trades with her if you start W. Try to get your level first so that you can kill her before she reaches level 2.

Gnar

Gnar is a ranged champ, so before you get your Q, they can poke you for a while. Be careful when CSing, as Gnar can slow you with his Q. Always pay attention to how long your cooldowns are. I recommend bringing Flash and Ignite.

Nasus

Nasus is quite difficult to deal with if he starts getting stacks on his Q. His ultimate is also very annoying. Try to push him back early game, but be careful since your cooldowns are very long. be wary of ganks as Nasus can slow you down by a substantial amount, leaving you vulnerable for the enemy jungler to come up and kill you. It is very hard to win if you die a few time since Nasus would have had time to get quite a few stacks on his Q.

Malphite

Malphite is pretty easy to fight once you have all your moves, but be careful early game. His passive can be annoying from levels 1-3 since your cooldowns are a bit long. Be careful of his Q, as he can steal your movement speed. Mid-Late game you should win since Malphite doesn't do too much damage with his abilites unless he builds AD. You can out heal his DPS eventually so be patient until you can.

Poppy

This one is a close Even-Major matchup. Poppy has a move which can block your E, which makes its hard for you to surprise her if she has a good enough reaction time. Other than that, Poppy isn't too much of a problem, although be careful around walls as her E can stun you if she hits you into it.

Sylas

Early game, you are not winning. SYlas has a faster cooldown than your Q and he also has lifesteal. Be careful until around level 6, your ult and all your other moves should be able to kill him if he isn't fed. Also, Sylas doesn't bounce when he uses your ult, so be careful if he steals it, pay attention to the sound and the damage you take.

Gwen