Only play this in ranked if you practiced a lot with it before, it is not safe at all when in-experimented, and people will probably dodge, you can't blame them ^^

This guy is an amazing AP Carry, but his biggest problem at this role is his dependence on skill shots, but if you want to mid with Ezreal, don't go AD, silly, this guy has more candies in his bag.

It indeed can be very hard to land a successful combo on an enemy champion sometimes, and it is frustrating, clairvoyance will help a lot, but patience can do way more.
Flash should be used to disengage if you made an error, as you can't afford to die with Mejas's.

In the early lane phase you should only spend your mana to harass (mostly with W), and if you are heavily pushed to tower, creep kills (with Q).
When you get "Tear of the Goddess" your mana pool has vastly expanded and harass now also benefits to your mana, so land some Essential Flux on your opponent whenever it is possible, without ever committing with E when unnecessary (wait your jungler ganks for that, or use it to dodge strong spells or the enemy jungler).
An excellent point about your W is that it doesn't damage creeps and instead go through them, giving you many harassement possibilites without pushing at all.

Sheen will help you deal mixed damage, making you even deadlier.

Eventually replace Mejas's by a Morellonomicon if you don't feel like being able to get kills that easily, Cooldown reduction & Ability Power are important.

You CAN always deal extremely high lethal damage, but accuracy is important.
Only use E to finish off Champions as a safe bet, you probably guessed that you are squishy, this E saves you from aggressive enemy flash-ins, along with your own Flash.
Don't forget your Q reduce your cooldowns by 1 second (summoner spells not included).

In the late game, you MUST use your Seraph active, as the shield can save you in many cases, remember when you die to never rage, but analyze your death.

You don't need defensive items as your mobility is incredible and your spells range dont require to put yourself in danger.

What is dangerous:
- Unseen bushes and surprise effect.
- Hard crowd-control effects (stuns, snares, silences, knockbacks).
- Assassins, you are their main target.
- Not being focused and missing all skill-shots.

That's all for now,
don't forget to have fun !