Well this is my first guide, I hope you find it useful, this guide is not so different from other anivia guides, Ill just share my experience with you.

This build is focused on harassment and Survivability and some of you may be asking: Why? Anivia hurts like hell!!! Well actually she HURTS like hell but with other builds that give me more AP I realized that on mid/end game I got focused a lot and got killed without even being able to cast anything so that's why I started to look ways of having more HP and decent DPS.

Well these are the ones I've been using for a while but this isn't carved on stone.
Flash: well this is a good choice because you can escape pretty fast, I also recommend to use your Q, W and R spells to escape they are both offensive and defensive spells it can save your life and your team's.
Clarity: I personaly love this one because I dont need to buy anything else than meki pendant at beggining so I get an extra of 85 gold that I can spend on tear of the goddess to get it faster.

I think Flash is a MUST and clarity is optional but you can suit yourself as you want.

[*] Meki Pendant : well this one is because you will need mana regen, you'll need all the mana you can get for successful harassement.
[*] Tear of the Goddess : like I said YOU NEED MANA! before everything else you need to secure mana so GET THIS!.
[*] Boots of Swiftness : some of you must be thinking why get these... well it's because with these you can stun the target with Q at large distance and punish him with your E without letting them to escape your E and also the 1 extra movement can save your life from your nemesis: Master Yi, Xin Zhao, Jax, and heroes like these NEVER specially Xin Zhao and I mean it NEVER face him alone, you will get raped unless they suck.

So your core build Will be:
[*] Boots of Swiftness
[*] Rod of Ages
[*] Archangel's Staff

The rest is optional but from this point you will have decent HP and a lot of mana, Clarity is also useful for you because you will be spaming your R a lot on minions to farm a lot so until your archangel caps it will be useful, even on endgame situations it can give you the last mana you need to kill someone or to run.

Early Game:

Use your Q to harass all the time, try to stun and punish with E, repeat this and you'll prevent your enemies from farming peacefully and you might get some kills, use your range to your advantage! Rape Annies, Ryzes and other closer rage casters. Also Ashes should be easy to handle.

Get W, its useful to explore bushes and it can save your life from ganks or it can get you those secs you need to use your E again and kill. Always use it after R to do a little of more damage with it.

If I have little HP I normally don't B unless it's really necessary, I do this so the other champions will be temped to tower dive, If they do this STUN THEM, you will be alive because of your passive and they will be dead or about to die, this is when you flash and kill.

Mid Game:

If you follow what I said ppl will start to hate you and focus you xD if you get fed OMG kill because you will have your core build up and can start having more AP wich means more kills.

End-game

Use your W to split enemy team on jungle fights or to prevent them from escaping, STAY AWAY from the heart of the fight if you can, always stun with Q, E, R and then W, fall back a little then when you got your E again kill.

Who should be your targets:

At some point of the game you will be able to do 1 kill come with some characters:

Ashe
LeBlanc
Lux
Orianna
Teemo

Champions like these have little HP so GET THEM. They will be free kills

This is very easy, just spam your R, get a lot of gold! xD Anivia is the perfect farmer.