The Tree Sentinel is only really hard when you’re newer to the game. Once you’re a lot more familiar with the game, Tree Sentinel isn’t nearly as tough as he was when I first fought him. For instance, I’ve beaten him at lvl 1 as the Wretch class with all base unpgraded gear and or course completely solo and without Torrent as well. Just takes patience, learning his moves and patterns, and how to properly dodge them and outspace him. It’s actually a really fun fight at lvl 1 once you get better at the game.
As for the DLC, I don’t think leveling up is necessarily the answer. I’ve seen people at super high levels (like levels 300+) still having issues. I do agree that the first boss and/or the DLC as a whole is like the Tree Sentinel in that you can’t just bum rush it and expect to not die… a lot. As long as you learn from each mistake and death, and have the patience, every boss in Elden Ring can technically be defeated at level 1. And there’s several people that have done it. And also some that have done it at level 1 without ever taking a single hit! Of course, these are streamers/content creators who essentially play the game for a living and can dedicate 8+ hrs a day to it, but still… lol