Leona is currently one of the Newest Tanks in the game. She is very strong at mid/late game,Leona offers alot to her team, 3 different stuns which is either 2 single cc or aoe cc due to her ulti . She also can be built two different ways from what i can see either pure tank or hybrid AP off tank.

1st build=standard tank masteries except i picked up strength of spirit for some extra health regen

2nd build=standard caster masteries except i picked up improved flash and ghost

1st Build
Greater Mark of Insight = over all good for leona because all her abilities scale off abiliy power

Greater Seal of Resilience = more armor less damage early on

Greater Glyph of Warding = early game mitagation against magic attacks

Greater Quintessence of Fortitude = because more health is win

2nd Build
Greater Mark of Insight = over all good for leona because all her abilities scale off abiliy power

Greater Seal of Resilience = more armor less damage early on

Greater Glyph of Focus = more spamming means more stuns and more cc

Greater Quintessence of Swiftness = because she is a little on the slow side

Other Good Choices
Greater Seal of Evasion = chance mitigation

Greater Seal of Vitality = health per level awesome for late game

Greater Quintessence of Insight = better overall damage output

Team Fight
Pop then and followed by on the carry

1v1
Pop use and and followed by another

I would say leona is a good buy for a tank/Ap off tank. She does what she is supposed to do tank aka soak up damage and make life hell for the other carries with stuns. both build are viable although her ap ratios are a little on the low side with the build you make up for it with stuns