Unlike in summoner's rift or twisted treeline,in dominion,Leona is very effective.
First for all,the map constitution allows Leona to use her QWE combo and running away afterwards.
For this reason,an AP build is more effective.
Then,her ultimate is no longer of an handicap anymore:
-It has a real big range and is AOE,going along with dominion's map constitution (big map/vision of ennemys)
-The speed buff in dominion,as stated above,allows Leona to quickly use her W/E/Q combo (which means activate buff/Zenith blade a target/Stun and run away before your buff wears off).
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For these rasons,an unlimited harass coming from Leona and an escape mechanic working on both you and your allies gets Leona to be the most efficient in Dominion than in other maps.
Ultimately,her burst is pretty cool...and since you can repeat it...it's fun see? Cause you take no damage in return.
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Skill order:
Stun for reduced cooldowns,Buff (amor/MR) only level 2 cause zenith blade's coldown is more important.Follow my skill order and you get to have pretty muich all skills on same cooldown.If I'm mistaken,just make it well yourself.
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Items;
As for items,I have introduced you THE Core build.As I don't see any sense to go building something else,do as you wish.
My games have ended when I hit these items.
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Smartcast zenith blade:
PLEASE DON'T! Don't use smartcast for Zenith blade,it's easyer to land without it.Same goes for ultimate.
2 skill shots aren't supposed to be too much of use to use them as smartcasts.

Summoner spells:
Exhaust/Ghost,due to their cooldowns is very useful.BUT remember to use EXHAUST to reduce their damage and NOT to chase them! (they hit you/Your ally and do big damage,use it).
Same goes for GHOST! Use it to INITIATE a teamfight! What I mean here,is position yourself for a perfect Zenith Blade (hit more than 1 target) as it does AOE damage! And remember,you're built AP. It is what makes the difference! Zenith Blade is your key for the win. (Note:I don't mean there early game,but its still useful :)

Well folks,have fun.I promise you lots of fun,much burst damage and taking no damage in return.Use speed buffs and heals from dominion and you won't probably have to go back to base.And don't be afraid of going in a 1V3 if you have your ultimate up and want to harass a bit someone.(Use ult to run away,or ghost...spd mobility boots wont make'em chase u)

Edit:Oh dang,they want 5k words lol...Ok guys let's end with

FAQ
Isn't Leona supposed to be a tank? Why build her AP?
Dominion's map allows her to run away without taking any damage.And in case they have 2 feets instead of 3 like you,(yeah guys Leona is pretty old,she doesn't look like una mujer :( ) your ultimate is here to delight them with memories of their mummy persuading them to go back home,NOW.Oh and in case you didn't understand,You initiate from the jungle (middle dominion map idk what its named) , you dont stay in lanes.You must somehow 'Gank' which means they musn't see you before you initiate.And in case they did,just blind them.

Leona has a blind? Err,... I was just trying to be poetic,blind them with your LIGHT!!!! Or you can seduce teeemo and...Oh dear,what has LoL come to? TEEMO NEEDS MORE SKINS!!!!! GENTLEMAN TEEMO FTW ¼

3 AP tomes? Wtf're you doing?
Trust me,these 3 AP tomes will allow you to 'kill' instead of 'assisting'.And in case you 'kill' you are rewarded by getting rid of these tomes. With one stone two birds heh.

Is it 5k words yet?
Well.............................................Perhaps if......................................I spam question marks???????????????????????We'll get there?????
Oh noes,plw dont ban me! Ima remove it laterz.

Why don't you do like the most,Use ===========================To separate sentences?
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Oh,never thought about it! Good idea.Wait i forgot to use them above! Edit:Fixed

Are you trying to be funny? You're not.
:'( I knew Leona wasn't laughable at :'( I should've picked Teemo!

Can we return to FAQ? oh sure.

Bottom or TOP?
Oh sry.Top of course.Leona is effective in bottom but its not worth all the assists you get in top.As you have AOE skills,you will get many many many assists.

So do you refuse to auto attack?
Well,its better to return in the middle and anticipate their moves than wasting time chasing them.(effect of the surprise)
===========================(sry Pals ima remove it later when I get things to write)==========================

Do you think tank build are viable at all?
When your char is a semi caster who can run away without any problem AND has long skill CDR,I don't think so.
You're too useless then.

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No tenacity at all?
Nope,it is countered by your ultimate. If they get you low and try to chase (which CC can afford) you can just run away due to ultimate's OP range.
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Why do you have to cheat in order to achieve 5K words?
Tell this to Obama: No smoke no inspiration.
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