By far my weakest role but I am currently working on it as I have been stuck playing it a few times and the results were not pretty. Last game I was trist/morg vs leona/vayne. I had a very hard time csing early without getting the all in from leona/vayne. Leona sure sucks for an ADC with a short range. Who is a good adc pick against that combo? I was thinking potentially Sivir?
The thing about leona, is that she can be super oppressive and hard to deal with. There are a lot of factors that can change this. The main two I would say to think about are your support and your positioning.
The first one, you may not have control over if its just some random player that you don't work well with. Someone like thresh, janna, alistar or braum is good for leona.
Because what you have to consider about leona is that when she grabs you, she is going in and that is her only option at that point. If have a support than can interrupt her grab, or make her pay for grabbing you by knocking her into turret (alistar) or can zone her adc from you (braum), her effectiveness significantly decreases.
Leonas, especially bad ones, will often put themselves out of position by grabbing you, try to take advantage of this. Pick an adc with a dash and then even if you are grabbed you ought to be too far away for their adc to damage you, and you and your support can mash leonas face for a second while their adc tries to get into the action.
Morgana should have been black shielding you everytime she grabbed you and that should have been a fairly simple lane, but that is sometimes the problem with bot, communication between random people is usually not too hot.
Also, I told my buddy this when he first started playing league and liked ADCs. The best champs to learn on in my opinion are Caitlyn and Lucian. Tristana is also not a bad one though, although I haven't even played her since the rework.
The reason I say Lucian and Cait is that they both have decent range, escape mechanics and poke. They will help you learn how to position while not getting you punished too hard for positioning poorly.
As a Platinum 1 ADC main, Leona can be very hard to play against. You're absolutely right when you thought about picking sivir as she has become my favorite ADC. Additionally, Sivir + morgana combo bot lane is hell for many FOTM supports (leona, blitz, thresh), they simply can't land anything on you. As far as learning the adc role, it's extremely reliant on who you're laning against. My play style totally changes against certain adc / support combos such as tristana / nami I would generally play decently aggressive early, whereas leona / graves you have to be very careful on engaging them.
I really started to excel as an ADC when I learned to farm flawlessly, especially under turret. I can't stress it enough how important CSing really is, it can easily win bot lane for you.
Hope this helped!
