Zed receives bonus effects from attack damage items he purchases, so armor is super effective against him.
After Zed uses his Living Shadow, he is at his most vulnerable as his damage, slow, and mobility are both dependent upon it.
Zed'sShadow Slash can only slow you if his shadow hits you with it.
Saving energy and spell cooldowns until you use your ultimate will potentially maximize Death Mark's damage.
By quickly activating Living Shadow twice, Zed will send out his shadow and immediately teleport to it afterwards, allowing for quick escapes. This can be used to go over walls.
Use Shadow Slash when your enemy catches up with the shadow to slow them and prevent pursuit.
Living Shadow provides vision, which can be used to check bushes or scout over walls.
If you move too far away from the shadow created by Living Shadow you can no longer switch places with them. An arrow indicates the direction of the shadow from Zed and changes colors depending on distance. If it is green then Zed is close enough to the shadow to swap positions with it; yellow means Zed can still switch places with it but will soon be out of range while red means he cannot.
The skill also may allow you to catch enemies off guard after a small skirmish. Using Death Mark primarily for damage, do not forget that it places a Living Shadow at the location of the cast. Faking retreat after a kill, deceive your enemy by swapping places with your ultimate's shadow and use your Living Shadow to effectively give your Razor Shuriken 50% more damage output.
Three overlapping Shadow Slashes will apply a stronger slow than just two, however won't do any additional damage. If you already have a shadow up from your Death Mark, strongly consider saving your Living Shadow for escaping or chasing.
With just 10% cooldown reduction, the cooldown of Shadow Slash becomes short enough to land two of them before Death Mark's damage triggers. This can be used against enemies who don't consider Zed's cooldowns.
The first tactic is to place Living Shadow in a bush opposite of the direction you are running in, slow them with Shadow Slash and switch places with your shadow afterwards for a nifty escape trick.
The second is to place your shadow behind you and run in the opposite direction in an attempt to have them dodge an uncast Shadow Slash or Razor Shuriken, then cast Death Mark to fool your pursuer into chasing you while you escape with your Living Shadow and then your ultimate's. In some cases these strategies are way better than just simply using Living Shadow as a quick flash to safety, and might even ensure a kill.
Death Mark gives an untargetable state that lasts slightly longer than Hallucinate. Use it to avoid channeled spells such as Absolute Zero, Requiem, Noxian Guillotine, or with good timing, last-moment skillshots and autoattacks. (He can still be killed with DoTs or even die to another Zed's ultimate during this time)
Contempt for the Weak makes him very good at last hitting/stealing important monster kills especially in the jungle, but be careful when laning as if you abuse the bonus damage too much you might end up pushing the lane to your disadvantage.
An effective way to guarantee kills is to have deception as your ally. Letting a bruiser (an ADC/APC as a weaker bait might work occasionally) initiate on the enemy can ensure an advantage, often providing a good 'clearing up' for a shutdown kill or even an ace.
This tactic becomes extremely potent in late-game teamfights, as even though Death Mark's damage is primarily single-target, Shadow Slash and Razor Shuriken's damage remains AoE, being able to shred through carries and groups of champions with ease.
Zed plays as an AD caster, making attack damage, armor penetration, and cooldown reduction more valuable than critical strike chance and attack speed.
Getting Maw of Malmortius can be a good choice if you are being bullied by opponents with large amounts of magic damage.
While not the most viable option, Zed can purchase critical strike items and still be a dangerous threat. When building critical strike Zed, a great item to get is Infinity Edge. This makes Zed's autoattacks do huge amounts of damage should they crit, and potentially killing targets below 50% life when combined with Contempt for the Weak. While this item has the potential to make Zed extremely bursty, keep in mind that Zed still relies more on his abilities than his autoattacks for damage, and on many situations, items like Maw of Malmortius or Ravenous Hydra may prove to be better choices, with Infinity Edge being an option only if Zed is ahead and dominating.
A Blade of the Ruined King allows Zed to duel extremely well, as it adds extra single target damage that allows him to keep on top of his opponent as well as increasing the total damage when his Death Mark pops. The extra attack speed also allows him to take down towers quickly, and the active allows him to split push, knowing you'll be able to kill most of the things thrown at you.
The high AD and active from Tiamat/Ravenous Hydra can add significant damage to Death Mark, since Zed starts basic-attacking his target automatically. After the first hit, use the active to deal more damage, reset the attack timer and attack again. The combination can be as much as 300% of Zed's AD in a very short time. This item also includes a lot of extra wave clear for Zed, increasing his split push potential further.
If Zed's opponents have high amounts of crowd control, and he finds himself being locked down, a Quicksilver Sash/Mercurial Scimitar can be effective, not only reducing magic damage on him from his opponents, the upgrade gives him a considerable amount of AD.
As some games go, it is relatively more cost efficient to rush a Guardian Angel as the enemy players will be quick to focus on Zed in a team fight.
Randuin's Omen and Banshee's Veil are two good defensive items, but since they don't provide any offensive stats for Zed, they are situational. Either of them are a good option to buy if Guardian Angel is on cooldown, though.
After Zed uses his Living Shadow and Death Mark, he is at his most vulnerable as his damage and mobility are both dependent upon it.
Razor Shurikens deals less damage if they pass through other units before hitting your champion.
Escaping laterally, rather than directly away, from Zed when he uses Death Mark will slightly limit his ability to slow and pursue your champion.
On that note, be very careful when tower hugging at low health, as he can easily kill you while only taking a minimal amount of damage from the tower.
Beware that with a full build, Zed can potentially duel even full tanks. However, he is extremely reliant on Death Mark to do so; even against squishy targets, he relies on his ultimate to get a kill, as it's his ultimate that deals the majority of his damage. He will most likely use all of his abilities and autoattack a lot to do as much damage as possible.
If you have any hard CC, try to save it for after Zed activates Death Mark (and if possible, Living Shadow), as this will both reduce the time he has to combo the mark and give the marked champion a chance to escape.
If you are an AP champion, purchasing Zhonya's Hourglass can counter Death Mark; activate it just before it's triggered to become completely immune to the damage. The armor also helps to reduce Zed's damage.
If you are affected by Death Mark and a shuriken appears over your head, try to heal or shield, because that means that its damage is enough to kill you.