In early games always try to poke the enemy adc as much can with your q.

There will be 2 type of adc zilean will be against. We can differentiate them as enemy with dash or good mobility and enemy without dash.

1. Enemies with dash

At lvl 1 and 2

Difficult to make the bomb land on them. Try to poke them with aa mostly as when they come to take a cs as the last hit. This will be the key point to poke them. Make the timing perfectly to place the bomb on them when they come for a cs. This way its easy to poke them with aa and q.

At lvl 3

When you hit lvl 3, always try to slow the enemy with e first before landing a double bomb for stun. That way you can make sure you can land the stun on them. When you slow the champion, wait for them to make the dash, call out your adc to poke them so that they will make the dash as soon as possible to escape since they dont have mobility. When they make the dash, its your call. Make the stun and trade it wisely.

PS : Dont ever waste placing double bombs by misplacing it. Cuz zilean will be hungry for mana early game.

Example - Lucian, Vayne, Ezreal, Caitlyn,Kallista (High Difficulty)

2. Enemies without dash -

At lvl 1 and 2

These are the easiest laning phase for zilean. Its easy to place the bomb on them and it is easy to even aa them. We can predict easily where they be when they are going to take a cs. As much as place a bomb on them when they take cs or aa them.

At lvl 3

Hitting lvl 3 is the magic time for zilean. When the enemy adc comes for cs, slow them with e and then wait for some millisecond to predict where they move and place the double bomb on them. when you trade this the second time, its sure a kill or a flash burned.

Example - Twitch, Jinx, Ashe, MF, Jhin (Dodge his abilities while doing the combo), Sivir (Take out her shield and she is in dark)
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Mid game is important for zilean. We need to setup as many kills as possible for the team. At this time we should have finished our morellonomicon and got 45% CDR. At this time we can stun an enemy every 8 sec approximately, provided we maxed q first and e second.

Dont always wait for group stun. Take picks now and then cuz with 45% CDR we can get stun quickly. Even if its a tanky support doesnt matter, pick him with stun. A team without an engage is a team easy to win the the fights.

In team fight if you missed your combo for stun, wait for the next combo. Dont use the q alone again as it will come quickly before w. Stun is the important key factor for zilean to shine in team fights. He can even make a 5 man stun and its GG.

Late Game is highly important for zielan to be careful. Because in Late game, enemy adc can even 2 or 3 shot zilean with their item builds.when trying to make a combo always keep moving against enemy mid, so they misses their abilities. With the item build you will be tanky enough to take some damage and making time to land the stun. If combo misses, run run run....Against an adc, pray to GOD. Dont hesitate to ult yourself, cuz the team needs your STUN!!!