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1. Why AP Ezreal? Isn't AD a lot better?
Yes, yes AD does more DPS. But AP Ezreal is still viable and is a force to be reckoned with. Let's compare the two:
AD Ezreal: Faster mid game, high single target DPS, easy to use.
AP Ezreal: Stronger late game, high AoE spammable bursts... tricky to use.
It's far more vital that you land a lot of your shots as AP Ezreal, because since your regular attacks are now pathetic, you don't want to mess up your skills, as spammable as they are. Your biggest advantage is your incredible kiting and sniping abilities, you will want to hang behind your tanks and lay waste to the enemies from a safe a distance as possible.
That being said, AP Ezreal isn't always the best choice. You have to weigh against your enemies lineup as best as you can, if your team requires a lot of DPS to effectively win, then AD Ezreal is definitely better. AP Ezreal is a great AoE nuker, played more like a standard mage/glass cannon. He excels in teams with large AoE CC, Amumu, Nunu, etc, where he can just pop into the fight and spam shots at the enemies. If your team's CC is good enough, you can just spam your shots without even really aiming and land good hits (of course you should aim though). I'll go more into the actual game play later.
Magic Penetration Marks and Ability Power Glyphs are pretty much standard on most mages. The Mana Regen Seals... they're there because (AP) Ezreal drains mana way too fast. (I'll talk about the lack of mana items later.) Feel free to substitute some of the Seals for other things if you feel you have enough mana though. Flat HP Quints are there because Ezreal is freaking squishy.
Not much to say here. You can do em however you want but I find this most effective. The 3 points in Strength of Spirit really help Ezreal early game, and the other 6 in Defense really do help a bit. I don't really recommend an Offense mastery build because Ez really needs the Utility aspects. Mana is key to AP Ezreal, you'll want to be able to spam everything you've got as fast as you can. If you find yourself not needing Summoner Spells that much, you can allocate the points spent in Presence of the Master and Blink of an Eye other places. Up to you.
Ghost and Flash. Why? Like I said, Ezreal is squishy as hell. You die way too fast, it's almost horrifying. You have to maneuver in and out of fights incredibly well, know when it's time to fall back and know when it's safe to chase. Cleanse is of course a good pick when the enemies have lots of CC, other than that, I wouldn't take anything else (Clarity and Clairvoyance are okay, but I prefer the utility of Ghost/Flash/Cleanse.) If you drop Ghost/Flash for Cleanse, allocate that spare point in Utility to something more useful.
Ghost and/or Flash allow you to escape and chase incredibly well, which are two things that you will find yourself doing a LOT as Ezreal (AP or AD).
There aren't a lot of mana regen items on my Ezreal build, because no items are really capable of handling Ezrael's severe mana drain. That's why you have Presence of the Master. Grab the golem buff whenever you can, it helps SO MUCH. Virtually infinite mana, because Ezreal's spells are really cheap on mana, it's the spam that makes it draining. With it, you can spam spells left and right and not worry about running out.You should be more than able to kill the golem alone, although this is NOT advisable. Always try to recruit a teammate to help you speed things up, or just as a bodyguard/tank. It takes awhile if you just try to solo it, and you'll probably have to go back to base to heal after.
Start with a Sapphire Crystal and 2 Health Potions. The extra mana is enough to help you survive your laning phase, and it gives a nice boost to Strength of Spirit. Once you hit roughly 1350 gold, pick up Tear of the Goddess and regular Boots. Try to grab the Amplifying Tome for your Soulstealer too.
Grab your Mejai's Soulstealer (1235 gold, keep that in mind!) (crucial for AP buildup for Ezreal... if you can't keep the stacks... you probably shouldn't even be playing AP Ezreal. Sorry, it's harsh but true.) Then start working on your Lich Bane. Upgrade boots to whatever is necessary, Mercury or Sorcerer's are the best usually. And that ends your core build.
Everything else from this point on is up to you. Void Staff is fantastic if the enemy is stacking Resistance (don't get Abyssal Scepter, enemies typically won't be in range for the aura effect when you're sniping), Zhonya's is always a great addition to any AP Build. Rylai's Scepter is a great utility item for Ezreal, giving him a small CC to help keep targets in line, and some additional health. Will of the Anicents... eh, it's alright, comes in handy, but the other items are probably more useful. (Try to convince your allies to get it though =P)
Mystic Shot [Q] - One level early game is all you need. Use it for last hitting and harassing. Late game, just spam it to reduce cooldowns. It starts doing damage only once you get your Lich Bane (which is what makes LB a core item!)
Essence Flux [W] - Useless until you get at least 3 levels in it. Your basic nuke, you will spam this whenever it's available in fights. Don't forget you can still use it to buff ally attack speeds, it's great for tower/tors. Deals pretty nice AoE damage.
Arcane Shift [E] - Primarily used for escaping/chasing. Don't forget that you can shift over walls with it, despite the range not being shown as particularly high. Not a good idea to initiate/enter battles with this, the cooldown is too long and you'll find yourself stuck in the middle without a good escape (unless you want to burn Flash, but that's not quite worth it imo). If you have the golem buff though, the cooldown should be low enough (if you're spamming Q) to fight using this too. Just watch out for your positioning. Try to dance around the edge of the battles, shooting inwards.
Trueshot Barrage [R] - A pretty solid AoE nuke. Does a credible amount of damage late game once your AP is high. You can use it to snipe off enemies in other lanes early game but that's pretty hard to do. Late game, use it to open team fights, try to hit each enemy so you are awarded the assist for your Soulstealer at the very least. Don't hold it in reserve, the CD is short enough (and reduced quickly by Mystic Shot) that you can use it almost whenever. At 40% (MAX) CD reduction, the cooldown is a measly 60 seconds, assuming you're not spamming Mystic Shot (which you should be). The CD is easily reduced to about 30-40 seconds, or lower if you have a lot of easy Mystic Shot targets.
Early Laning Phase
Follow the item and skill build. Your lane choice is varied with Ezreal, both mid and side lanes are perfectly fine. Mid = more farming, Side = more fighting. Your call, based on the lineup. I'll be focusing on a Mid-oriented strategy though.
If you're being harassed badly, use Mystic Shot to score those last hits, or to counter-harass and drive the enemy back. If the enemy starts getting cocky by standing within his creep-line to avoid your Shot, it's a perfect mindgame setup to use Essence Flux with later. They probably won't see it coming, and get slapped in the face with a nice amount of damage. Focus on your farming though, and play safe. Harass when you can, and only when you can't be counter-harassed. Ezreal's pretty squishy.
ALWAYS CHECK YOUR LANING OPPONENT'S SUMMONER SKILLS. You do NOT want to be caught offguard by a surprise heal or ignite. That being said, if you harass enough, you might be able to score a kill on an unsuspecting enemy by Shifting into their face and firing off Flux and Shot. Make sure you do the math first though, add up your total damages and check their HP/Magic resistance to make absolutely sure you'll do enough to get the kill. It's a huge gamble, and if the enemy starts playing TOO defensive, you'll lose your chance because Ezreal can't really tower dive.
Flash is a good way to line up your combo if you can't quite make it (and if it's available...). Flash nearby the enemy, away from the creeps, then Shift towards the enemy, to make sure the Arcane shot hits him. Flux and Shot should be easy to chain off of once you're that close and the enemy is fleeing. Ideally you won't need Barrage to clean up, but if you do, go for it. The CD is short anyway. Just try to anticipate their juking.
Once you gather enough gold for your early items, go back and grab them. If there are low HP enemies (about half?) in either of the other lanes, go try for a gank, if not, just go back to farming for your Soulstealer. (Ganking is highly advised though.)
Mid Ganking Phase
By now you should be either close or have completed your Soulstealer, and are level 11-ish. Start ganking once you have your Soulstealer and rack up those stacks. They're integral to any AP build, especially Ezreal's. Ezreal lives up to his Champion Tag of Assassin, his burst nukes early game are quite easily capable of chasing and finishing off enemies. Just don't get too greedy and suicide.
Go for the golem buff if you can, hopefully get an ally to help you, then go gank and spellspam in the enemies faces. Ideally you get enough gold off kills to kickstart your Lich Bane, or at least upgrade your boots.
Late Ending Phase
Team fights are far more common now, and you should be nearing the completion of your Lich Bane. Initiate teamfights with Trueshot Barrage so that you can rack up the assists/stacks. Do NOT ever initiate by yourself. Wait for your team, let the enemies pile into a nice group, and let loose with Trueshot + Essence Flux. As I said before, save Arcane Shift for escapes/chasing, but it does do a nice amount of damage. If you're confident in your survivability and damage, you can try Shifting into a fragile enemy and blasting them with all your other spells for a kill. It's pretty risky though, but if you can take out the enemy's main damage dealers like this, you'll be a lot safer too.
Always put your safety first. Your stacks are integral at holding your AP up. Without AP... AP Ezreal is freaking useless. Stay a safe distance away at all times, sniping at enemies with your shots. Don't be afraid to get into the fight, but don't be an idiot and just waltz into their DPS's range either. Your ranged spamming is your strongest advantage. ABUSE IT.
Do NOT get involved in fights you know it's impossible to win. Of course, if your presence turns the tide, then you'd better help. It's a judgment thing. Ezreal isn't known for saving lives (anymore... stupid Essence nerf), unless maybe you have Rylai's, which could help your allies out of a jam. Your survival takes precedence over your allies', don't die for your allies, you're not the martyr. Ideally however, you should be doing enough damage to kill the enemies before they kill your allies (or at least force their retreat). Essence Flux's attack speed alterations can make a huge difference.
Rack up your stacks and KEEP THEM SAFE. Pro-tip: 20 stacks + Golem Buff = 40%/Max CD Reduction (this is why I have no CD Reduction runes or items other than Mejai's).
5. Final Notes
AP Ezreal is played as a sniper, stand in the back and lay waste to your enemies, don't get caught up in fights. Ultimately you're a damaging support. Don't get involved until you see all of their team occupied in the fight already, and you can go out and snipe them down one by one (or several at a time, if they're packed closely enough). I once walked from a 5v5 full-on team fight with a Penta Kill but all four of my allies dead. Occurrences like that are pretty par for the course when you're playing so defensively. People may call you a coward but that need not worry you as you laugh while you stuff magic down their throats.
This guide is ridiculously based on you being able to nab the golem buff. Without it, you'll be forced to make lots of trips back to the fountain (which you might have to do anyway due to Ezreal's low defenses). So do everything in your power to nab yourself that buff, whether you steal it from enemies or even allies (but please, for everyone's sake, at least ask them first. Don't be an ***.)
Once again, I'll say that AD Ezreal is definitely stronger one on one, but AP Ezreal is still viable and is still a force to be reckoned with when played right, and can even out-damage AD if the AoE shots work out well. AP Ezreal is a more standard nuking mage. It's a matter of utility. As I said before, some games are better played with AD Ezreal, some with AP, based on the team's lineup. Use this guide however you see fit.