So, while Senna has only been out for about a month now, i have about 50ish games combined between this account and my smurf. Both with high winrates and kdas .
So let's start with why Senna is strong. Damage wise, she is actually rather lacking in comparison to other adcs. Both her dps and bps if you compare it to a similar adc like Jhin or a conventional adc like Caitlyn. So why is her win rate so high at the moment if her damage is actually considerably low? It's simple, because of her massive range mid to late game, which if properly played, will usually be between 675 and 750, allows for enormous safety and the mvpsd she gets on hit allows her to reposition easily; in other words, she gets more chances to do damage in a fight in comparison to the usual adc. This paired up with the strong self-healing from her q and the low cd and root duration on w, makes her pretty hard to lock down.
Plus the teamwide utility that she offers is nothing to scoff at either and typically speaking, high utility champs in both bot lane and mid lane, have always had higher than average winrates.
So what can we do to better balance out Senna?
Let's start with her range. Her 600 AA range is especially oppressive for support senna, as she can poke and harass much easier than ad senna due to her having to less worry about cs and putting herself in potentially hazardous situations, the 600 range in lane isn't as big a deal for ad senna, because the cast time on her autos and how close she is to the wave, often opens her up to retaliation. But, she will eventually scale up to a very large range, so I think lowering her AA range to 575 is the first thing that needs to happen. This will mean that she has get to 20 more stacks to make up for the nerf, which pushes her further to the late game where Senna typically falls off due to the huge crit penalty and lack of attk speed.
Next, the self heal on her q is way too strong for both ad and support senna. Even a champion with a similar functioning heal like nami, gets reduced heals when someone else is targeted, but this is an even bigger problem for Senna, as she becomes an adc with a huge self-heal, something that all adcs do not get w/o spending money. So, I say that the self-heal should be reduced by 50 to 75 percent. The range on the pass through damage could also be slightly nerfed. However, her q mana cost needs to be decreased as it currently costs 110 mana which is kinda ridiculous now and even more so if the provided nerfs go through. I'd say to like 90 mana is fair. Her W could probably use a slight cd increase, E should probably be changed in some way shape or form as it's basically a get out of jail free card for her allies and senna herself, since they can't be targeted while in it, but at the same time, she needs to offer more than a heal weaker than rakas and a root that can easily miss, so idk how Riot wants to handle it, I just know it will be a big issue once Pro League starts up again.
I think her ult is fine as is, both the damage and shielding are fair.
I think these nerfs are fair and am interested in what you guys think. I do have some compensation buffs in mind if Senna's win rate drops too much following these nerfs as well.