Since you don't believe it, I'll do the math for you.
200 armor.
The formula used to calculate damage is thus: Damage multiplier = 100 / 100 + Armor, Armor >= 0
After applying armor penetration and the extra armor from your W, you have 260 armor. Damage multiplier = 100 / 100 + 260 = 0.278 This means that all of Rengar's damage is multiplied by 0.278.
Rengar has 96 base AD at level fourteen. With his items, he has 271 AD. Let's assume his combo is R-eQ-Hydra-Q-W-E before hitting the ground. This combo is perfectly achievable in under one second. Rengar's empowered Q increases his AD by 10%. This would give Rengar a total of 298 AD. eQ (200 + 144) + Hydra (298) + Q (150 + 358) + W (50m) + E (250 + 123) = (1573 * 0.278 = 437) Leona, at level twelve with a Sunfire, has 1883 health. Before he lands, you are at 1446. That's roughly a quarter of your health before you're able to retaliate, assuming you already had your W activated. This is also before he lands. He also has five more ferocity generating on top of the three he just gained. It's possible to activate three empowered abilities in under a few seconds. This is also not taking into account runes and masteries for either of you. In reality, Rengar most likely did more than this to you. I am not taking into account the 6% penetration from masteries (10 arpen at 200 armor), +5% bonus AD (9 AD), +3% increased damage, roughly 10 extra at level AD, and another 15 AD from runes. In reality, he could have probably brought you down to half with that combo.
If you were isolated and you had to run away, I would not be surprised if he were able to auto you to death as well as keep you slowed. He has significantly higher movement speed, as well as movement speed and attack speed steroids.