Ekko is significantly weaker when his ultimate is down. Watch for the trail he leaves behind to determine if Chronobreak is available.
Ekko's stun zone takes 3 seconds to arm. Watch for the image he creates on cast and try to guess where the zone was placed.
The second hit of Timewinder does more damage than the first; try to avoid it.
Ekko is mainly played as an mobile burst assassin, with high AP and CDR, to delete squishes while remaining a slippery nuisance. But due to his 'sticky' kit, he can also be played as a "tanky mage", combining items like Rod of Ages, Iceborn Gauntlet and Spirit Visage.
All of his skills have great synergy with themselves. Q+W or W+Q to slow/stun the enemy and land the latter perfectly, E+W to get to the sphere faster, Ult+W for the AoE stun and damage etc.
It is possible to proc his passive on every damaging spell. So running and using Q for a double stack and then Ulting back on the target is a risky but rewarding way to one-shot most champions.
Q is a really good farming tool as it has a low cooldown and decent damage. Be careful though as spamming it will drain your mana and may also ruin the minion wave without enough AP.
Never underestimate your W's passive; when enemies are low health, it does surprisingly high damage and can help you finish off targets easily.
It is important to stack a lot of AP as it will go to both your damage and survivability from your Ult healing and W shield.
Ekko is a great chaser and runner as he has a movespeed boost on his passive, two slows (Q and W), a stun on his W, a dash and blink on his E, and another blink on his Ult.
For the fastest damage combo, throw out your Q and E your target as it's coming back. This will proc your passive and result in a near instantaneous burst of damage.
Ekko's split pushing capabilities are quite good as he has great waveclear with his Q and his E damage affects towers. Due to his mobility, he can also escape easily when enemies come to try and stop him.
For the best use of Timewinder, try to zone your lane opponent against a wall as they will have less space to move and dodge both parts of it.
Don't waste your W and Ult as they have relatively long cooldowns and are important to your survival. Time them both right to utilise as many of their effects as possible.
For Parallel Convergence, at least try to get the shield to mitigate retaliatory damage from your opponent.
For Chronobreak, the healing and repositioning components are most important. Don't worry too much about landing the damage as your kit still has plenty of damage without your ult.
Push to the limit while fighting before you Chronobreak. This will allow to heal as much as possible while baiting enemies into taking the damage from it as well. However, this is not always the best choice, especially in team fights, as enemies may burst or CC you before you can ult.
Always try to proc the third hit from your passive when possible. The bonus speed and damage can potentially win a trade without much effort.
Phase Dive has strange synergy with Rapid Firecannon. The item's damage will proc first than the damage from E but the bonus range will still be applied.
Phase Dive consists of a dash followed by a ranged blink-auto attack. The bonus range is still applied as the normal method of range calculation. In other words, the range is calculated from edge-to-edge (your champion's position to the edge of the range meter). In addition, the edge-to-edge portion scales with the champion's size (and Rapid Firecannon), which means that a larger Ekko will have greater range than a smaller one.
The best keystone for Ekko is Thunderlord's Decree, as it can proc with really well with his passive. Thunderlord's Decree can proc with the 2 Q and E, but, if you use only one Q damage and E, it will proc as well, since the E damage is physical+magical.
Vampirism from Ferocity Tree can be a good choice for extended fights and trades.
If you have free points, spent them in the Resolve tree to obtain the bonus shield and healing.
Deathfire's Touch is a good alternat spell, despite not having a perfect synergy with his skillset. But when the enemy team is hard to get to close combat or they are too much tanky, you may have it since it can really negate some health regen from enemies, specially on the lane phase. It can full proc on his passive, and combined with Liandry's Torment, can really do the job against tank enemies.
For more tank playstyle, pick Grasp of The Undying.
It is necessary to stack high amount of ability power, Ekko got great scalling rates in every single skill. Passive's got 80%.Timewinder back and forth damage totalizes 90% of ability power scalling rate. 150% ability power from W's shield. Phase Dive got 40% (plus the damage from Lich Bane) and 150% from damage scalling on Chronobreak alongside 60% of healing.
Catalyst of Aeons can be a really good pick, as was mentioned, Ekko strength's is focused on constantly damage alongside with a good cooldown reduction, especially when his damage output is still average to low. The two passives works really well with him and the two final items benefits his kit really good too.
When facing too much physical damage, Iceborn Gauntlet will be a really good choice, making even more easily to land your combos.
Spirit Visage will boost significantly stats for surviability and enhanced cure. Perfect for extended team fights.
Luden's Echo works really good with all of his kit plus the bonus move speed and crucial for an assassin playstyle.
To maximize your damage, you will need to constantly keep close to your targets, which makes Abyssal Mask an efficient buy.
Nashor's Tooth boosts your kit really well. Ekko got a good synergy with attack speed, especially combined with his passive from Parallel Convergion. Alongside with the bonus AP and cooldown reduction, it will make your split push really strong.
Despite not being too much good for his essential attributes, Wit's End enhances his close auto-attack combat, reducing magical resistance on every hit, potentializing all of his burst.
Cooldown reduction is essential. Ionian Boots of Lucidity can be a good choice, as it can even reduce the cooldown from your spells, making even more strong for hard engages and constantly presence if you are in top lane or using Teleport.
Even building tanky itens, Ekko is still vulnerable to Crowd Control since great part of his skillset is focused on keep moving around, when necessary, buy a Mercury's Treads to always ult before dying.
Zhonya's Hourglass works good with the time of your ult. When in stasis, the shadow from Chronobreak will still be active, so it can catch enemies of guard.
Hextech Gunblade is a really good alternative item. Combined with more on hit itens with bonus effects, it can upgrade your melee combat as well.
Frost Queen's Claim is another possible build path on Ekko. It's AP and CDR makes it a good buy as it plays to the stat's that he needs, as well as giving him large gold income, and active that can be very helpful for catching enemies in your W circle, and the Move speed passive that synergies naturally with your Passive movement speed. Late game, proccing your Z-Drive Resonance will turn you into a speed demon.
Like many other assassins, Ekko is vulnerable to crowd control. Snaring or stunning Ekko before he can deal heavy damage can heavily cripple his potential in teamfights.
After using Phase Dive, Ekko is even more vulnerable to heavy CC and huge bursts. This situation will likely cause most Ekko players to burn Chronobreak just to survive the incoming burst damage.
While Ekko is a threat on his own, His kit is to primarily for 1v1 dueling and start fights. Dodging Timewinder, or Parallel Convergence can prevent Ekko from closing in on you easily.
Baiting Ekko in using his Parallel Convergence into heavy ambushes can be a good strategy. His stun and shield is on a long cooldown and your team can close up on him and kill him.
When Ekko is retreating with low health and his hologram is following him, be wary. It means his Chronobreak is available and he can instantly turn the fight around with its healing and heavy AP damage.
When your team is chasing a low health Ekko, If you notice Chronobreak is ready, Launch skillshots onto his hologram. Ekko players tend to do this rather than running away to attempt to turn the tables. Its better if it is a heavy CC. Your team can catch up and kill him.