This build has worked best for playing Leona in Dominion. She will not do as much damage as other characters like Xin, but naturally she isn't meant to. She needs to deal damage in Dominion to make her viable, with this build there are the more pro's in my opinion.
-Able to initiate/dive/tank/push enemy off of their turrets amazingly (will be explained later)
-Burst CC during a 2v2 or 3v3 fight which totally wrecks the other team if they are built heavy damage and low sustain
-Able to deal damage so that after the CC burst, you still are a threat
-Able to push creeps very fast (a lot of champions waist time pushing creeps, Leona can do it off handedly with her shields burst)
-Her skills are burst, they do a lot of damage to characters who don't have many hp items, but they aren't equivalent to a character as mentioned earlier like xin or jax.
-No character is amazing at 1v1 (both full hp) on top of a turret without creeps with you.
You could go Garrison but Ghost is a lot better.
Garrison helps a lot in protecting your turret, but ghost will in the long run help you out more on Leona in Dominion then Garrison.
Ghost will help you if you went bottom to help in a fight or for some reason are in the jungle, to be able to get to the top fast to stop someone from fully capturing the top.
Ruby Crystal, Boots, and Sword
Upgrade when you end up base next, to Phage
Get Mercury Treds
Get Sheen >> work to Trinity
Get Entropy (If you didn't need to go base for a long time and have enough for BF Sword, then get that first, other wise get Ruby Crystal > Phage and then finish Entropy)
Now work on Infinity Sword (You could get a Gigantic Belt if you wanted, I've done both, I prefer working towards Infinity Sword.)
For the runes, they are straight forward.
On Leona, you always want the magic penetration. Her W's AoE burst is an extremely important skill on Leona, and this little bit of MPen eliminates most of what people get in Dominion for Magic Resist.
The Magic Resist and Armor scaling or not scaling due to leveling is your choice. I prefer having early on some solid armor to start out with her and don't worry too much about magic damage until later on when AP people in Dominion actually are doing a lot of damage.
It is necessary to get either MR and Armor runes scaling or not. The reason is because Leona's W gives her a lot of MR and Armor. This means that she doesn't need to waist money on armor. Even doing 5v5 summoners rift, players don't get a lot of armor on Leona. By setting up with both MR and Armor runes, you assure that you won't need to worry about extra armor during Dominion with this build.
(The dodge runes are because when fighting some champions it helps having a slight change of dodge. Luck can be on your side sometimes when defending, taking a turret, or just doing a normal fight. These can be changed out to eliminate luck for more armor runes if you wish.)
Why AP Runes?
The Greater Glyphs are really up to you, but you want to do damage with Leona, and you aren't building her AP. Why? Because AP in the end on Leona isn't that great. However, having a small boost of AP with the MPen from runes will sweeten Leona's W AoE burst.
Leona just isn't going to kill people with her ult or her Q. It just doesn't happen really. Her W though kills many people, and will be used a lot. This small boost to her W specifically is the best.
For this build, Leona's runes are basically set up in such a way to eliminate using money wastefully in game: Little Armor, MR, MPen, and AP are all this item build are missing which sweeten Leona, but in the long run won't help her a lot if bought excessively in game.
The purpose of Leona is to take a turret and be viable at holding it or pushing with the rest of your team to get another turret dealing enough damage while taking enough damage to do both. Leona rolls hybrid. Generally I see Dominion as characters who can hold a turret, those who can kill other champions, and those who can take a turret. Generally those who can hold a turret can take a turret with another champion. Most people just play those who can kill other champions in Dominion which are important but a team NEEDS someone who can jump in and tank a turret while dealing damage that they do more then just soak up turret damage.
So with Leona, as long as you have someone else with you, you pull yourself into them, shield stun, with shield up. The shield will go off, and you are taking the turret damage. Soak up damage till you're almost dead and then decide if you need to run from turret to live since you're almost dead or not. By this point your team should have scooped in and killed most of the people, with you tanking turret and having handed out CC.
Some people are able to 1v2 defend a turret. If you are in the 2 as Leona like this, they won't be able to defend it 1v2. The turret will get soaked up and you will CC and hurt them enough to almost kill them yourself with your ally champion doing the overkill on them.
Please: Don't build tank Leona or AP Leona in Dominion...Tank Leona can be okay, but so can tank Galio in Dominion....Never dying but not dishing out enough damage and NEEDING team mates always by your side. (And no one should ever play AP Leona unless you're trolling) This build will let you be more free which is needed in Dominion.