Summoners Spells: Heal, Flash/Exhaust

Poke early with your stun. Use shield of daybreak followed by Zenith Blade, after commencing fight, hit Eclipse and ult Solar Flame only when they are stunned and you know they will get hit. Remember that with Leona's passive, if you can do damage to the other team early the buff will greatly help the team fight.

Leona's shield is the greatest thing since sliced bread, since it continues to stay on her if it harms anything upon exploding, definitely max her shield first. After that priority would go to her Q (stun), followed by her E.

Don't be afraid to ult at a distance if it means slowing someone down and saving a team mates life. They buffed her range so you can drop it from quite a ways away.

The idea of going Philosophers stone and Heart of Gold early is so your support can focus on creeps and yet you still get plenty of gold. This combined with the extra gold from your rune page and you should be able to get the build pretty quick. The items listed in the build are reasonable to get to. If AD begins to carry early on a good counter for this is after the Sunfire Cape, is buy the Armor Boots and take philosophers stone into an Elysia's instead, inexpensive upgrade that still gets you the tenacity you need. On the contrary, should AD not do as well, don't waste your time with Thornmail, instead just go Warmog since Force of Nature's health back is a percentage based off of total health.

I suggest moving the Reverie and Omen items to spots 1 and 2, and getting into the habit that after you combo to engage a team fight (E>Q>W>R, or if you feel pro and know you wont miss W,Q,E,R), immediately pushing 1,2 to give the buffs early fight before anyone dies. I always end up forgetting about my actives, so I usually just spam them at the beginning while Leonas passive buffs are still on the opponents and watch em drop.

Ultimately, farm as much as possible when not needed in a team fight. If you can get to the end of this build, Leona will take a ton of damage and still stay up.