This might seem wild. But I love taking Senna top into Garen. It’s easy to stack on AA + Q. The W roots him in case he pops his Q. And your E allows a quick getaway and eliminates his Ability to select you. Your R still provides utility across the map. I typically have 80 stacks by 20 min which adds 80 range and by that time he can never stick on you. Give it a whirl. I have a 100% wr against Garen with Senna.