Bit of a LoL player myself, Diamond IV on my main and Plat V on my smurf I can tell you now that Leona as an AD works much better than AP Leona. Her skills may scale off AP but her skills aren't on a low enough cooldown to repeatedly spam them. Also take into account the meta at the moment is mostly tanks/high burst (Mundo, Shyvanna, Jinx, Ziggs) and Leona's "AP burst" isn't enough to warrent her actually building AP. However, the amount of CC she has does allow her plenty of time for auto attacks in between all her skills so building AD is much more beneficial to you should you want to play Leona as anything but a support (jungler).
You really want a cheese lane play Leona Volibear Bot, bot take the summoner spell exhaust. Voli initiates and flips, leona stuns said enemy champ voli exhausts after stun wears off you'll burn flash and barrier if you have ad reds with armor yellows and MR blues. At level two you will get a kill. Leona spears, Voli flips, Leona stuns, Voli Bites. You will win bot lane and in the current meta, you can carry with two tanks bot lane and no adc. Went on a 10 game win streak with my friend in ranked with it.