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For this build, you will start to get a little tanky because of the health you get from ROA, however you will still deal out immense damage... You can also substitute Wits end for something like Nashor's Tooth. I like the added attack speed on Ekko because it makes it easier to proc his passive., plus add magic resist and 42 bonus magic damage helps a lot considering he is a melee champion. The last item is completely up to you, however if I am ahead I do like to build will of the ancients because of the spell vamp. Another good item is Morc Book because of the added CDR Other options include Zhonyas, Rylai's, Trinity Force (Speed plus sheen, you can substitute for Lich Bane) Liandry's and so on. My final build order is usually Abyssal Scepter, ROA, MP Boots, Lich Bane, Wits End, and situational Item

Against Ad I like to build Gauntlets. After using an ability, it adds an extra slow, which is really helpful with his E which increases your Auto attack range, plus the 10 CDR is really nice. Sometimes i will build Morc Book for more CDR and ability power, but usually I'll go with something like Hextech or Rabadon's.

Z-Drive Resonance: Ekko's damaging abilities and basic attacks build up Resonance stacks on his enemies, lasting up to 4 seconds. At 3 stacks the target takes「 15 + (12 × Ekko's level) 」(+ 70% AP) bonus magic damage and is Slow icon slowed by 40 / 50 / 60 / 70 / 80% for 2 / 2.5 / 3 seconds. Z-Drive Resonance's bonus damage will apply spell effects.
Z-Drive Resonance cannot affect the same target more than once every few seconds. Against champions, Ekko also gains bonus movement speed equal in strength and duration to the Slow icon slow. Take advantage of the speed boost from the third stack. you can use it to chase down enemies or to run away.

TimeWinder: ACTIVE: Ekko throws a device in a target direction, dealing magic damage to all enemies along its path. At maximum range or if it hits an enemy champion, the device slows to a stop and expands, creating a Slow icon slowing field.
OUTGOING MAGIC DAMAGE: 60 / 75 / 90 / 105 / 120 (+ 20% AP)SLOW: 32 / 39 / 46 / 53 / 60%
After a short delay, the device contracts and rushes back to Ekko, dealing additional magic damage to all enemies along its path. Add two stacks. Use this wisely

Parallel Convergence: PASSIVE: Ekko's basic attacks deal 5%「 (+ 2.22% per 100 AP) 」of target's missing health as bonus magic damage (Capped at 150 vs. minions / monsters) versus targets below 30% of their maximum health.
ACTIVE: Ekko calls upon an alternate version of himself that bats a device to the target location. After a total delay of 3 seconds, the device expands into a broad sphere that Slow icon slows enemies by 40%. Enemies do not gain vision of the target area until the last 0.5 seconds.If Ekko enters the sphere, it detonates, granting him a shield for 2 seconds while Stun icon stunning all enemies inside for 2.25 seconds. Ekko can detonate the sphere while dead and while in Stasis icon stasis.

Phase Dive: ACTIVE: Ekko dashes towards a target location. His next basic attack within the next few seconds gains 300 bonus attack range (425 total range) and causes Ekko to blink on his target, dealing bonus magic damage. Phase Dive's bonus damage will apply spell effects.

Chronobreak:PASSIVE: Ekko is followed by a time-delayed hologram of himself that tracks where he was 4 seconds ago. The hologram is inactive while Chronobreak is on cooldown.
ACTIVE: After a short delay, Ekko blinks to wherever he was 4 seconds ago, dealing magic damage to surrounding enemies and healing Ekko for a value equal to flat amount plus a percentage of all damage taken over the last 4 seconds. This is a CRAZY ability. Take advantage of this when tower diving, or when engaging. Always remember to run around the map and back and forth, so that your clone is always way behinfd you. If you are being chased, and can turn around the fight, this is a great way to engage.

It can be difficult to land both stacks of your passive on an opponent with Timewinder, so a good way to do so is by engaging with your E first. Once you dash to them, shoot your Timewinder. You should be able to add at least 2 stacks with your E and the first part of your Q. If possible, try to add another basic attack. This will proc your passive doing immense damage plus giving you movement speed. After that you can decide whether or not you want to keep on fighting or run away with your bonus speed. Hopefully you can also hit them again with the return of your Q.

Parallel Convergence: This ability can make or break a fight. If you land this, you can stun your opponents AND get a massive shield out of it. DO NOT spam this ability because it does waste a lot of mana. It is best to use this when out of sight, so try to launch it when in a brush or from around a corner. They can see your clone shooting this ability so be careful where you place it. They can not see the actual ability until it lands. I like to place it behind them and pretending like I am running away from them. When they chase me I walk out of the ability and let them walk in. Once it lands I phase dive back in and land the stun, get my shield, throw my Q and go crazy. Usually by the time your W ends,your clone has caught up to you, so you can use your ult as a finisher move if you need a little more damage.

The rune guide that I have is when against an AD Champion. When VS an AP champion i like to take Health Per level seals and Magic Resist Glyphs