His Q W Q combo can allow him to stun a group of enemies which is highly beneficial in close-quarter combat. His scalings are quite high as well so he can deal a lot of damage through that combo.
His Ultimate R, when used properly, is one of the most obnoxious and overpowered abilities in the game. That combined with his E can make one of his carries nearly uncatchable and unkillable.
His E allows him to guarantee a Q W Q hit on a specified target. This can be highly beneficial in setting a death brush before a major objective fight.
Pre-six he is quite squishy, and easily killable if his E is on cooldown. Therefore, any champion who can catch up to him by dashing or blinking will be able to kill him.
His Q is quite telegraphed and he can be easily punished once his Q is down. Again, his Q can also be dodge by blinking/dashing away from the drop point of his Q.
He is pretty much a poke/damage bot when his Ultimate R is down. Zilean is inherently squishy so it should not be an issue to take him down once his Ultimate R is on cooldown.
0 - 15 min
Your goal throughout the laning phase is to poke and harass the enemy in lane. You have a lot of poke, so try to poke the enemy down as often as you can.
Play safe for the first few levels while you land poke on the enemy with your Q. Once they’re low enough, you can commit to an all-in.
Once you’ve hit level 6 and purchased key component items, look to harass and constantly push the enemy into their tower. This will allow you to earn turret plating and increase your gold income.
15 - 25 min
After your bot lane has moved to the mid lane, rotate to the bot side of the map or somewhere else on the map so you can continue to farm and gain XP.
Try and siege objectives with your team during the mid-game. You have strong poking tools which will allow you to siege objectives with ease. Try to poke the enemy down as much as possible while your team tries to take towers (or the Dragon or Baron).
Delay team fights for as long as possible while you harass the enemy and poke them down with your Q.
Stick with your team in the late game. Do not split up or walk away from them otherwise the enemy may try to pick you off or initiate a 4v5 fight when you’re away from your team.
Look for picks with your abilities. Picking someone off and then collapsing on them to take them down is a great way of winning a team fight and ending the game. Look for picks on isolated targets if possible.
Keep a constant eye on you and your allies positioning. You need to poke the enemy down with your Q and delay fights in the late game, but you need to watch your positioning as you move forward to harass. Do not get picked off by trying to harass and poke the enemy down.
At level three, Zilean will have access to all his abilities. This will allow him to play aggressively as well as poke the enemy mercilessly in the lane.
Level 6 offers quite a lot of protection to Zilean as it can allow him to counter lethal burst damage with ease. It will also make his opponent's job way harder than it is supposed to be.
Zilean's Ultimate R is going to be helpful in fights. Zilean's first item component will allow him to spam his abilities frequently, making life miserable for the enemy team. It does help a lot when dealing with champions who have a lot of burst damage in their kit.
At level 11, Zilean will be very hard to deal with.His Ultimate will keep him or a key target alive and will make assassins' lives a living nightmare.
At level 9, his Q will be maxed out and will allow him to deal tons of damage with a single bomb. It helps a lot when executing his Q > W > Q combo.
Zilean's damage and utility in any given fight are non-questionable. He can speed his allies out of trouble, revive them, or use them as a Q transport mechanism. Similarly, he can slow enemies, CC them, and allow his team to collapse on them.
At level 16, his Ultimate R will be completely maxed out. This means that he can use his Ultimate R way more frequently, which will be a source of nuisance for the enemy team.
This is the point of the game where teams are grouping which allows Zilean to use his Ultimate R to save his primary carries from certain death situations. It will also let him CC multiple targets with his Q > W > Q.
His Q will be on a low cooldown which means he can constantly poke and harass the enemy. Having more items will mean that he can chunk out enemies with ease.