The trees are indeed just for the purpose of obtaining crystal tears. These can be pretty cool if you actually use your custom potion and tune it to your build. If not, they just provide a boss fight and some runes. Speaking of: i always fight bosses on foot. I'm not a fan of the horse as a mechanic and I never really think to use it in fights. I find the tree guardians pretty manageable on foot and i realize fighting on horseback can some with some drawbacks if you fight an enemy that can actually handle it. For instance, you've been pretty vulnerable to combos. If you got hit once with a heavy swing, the horse flinched and you were unable to escape the follow up stuff like this just doesn't happen on foot since you can just roll. But then, you cant dodge attacks simply by moving as well on foot so it's really a trade-off. I think you may wanna give bosses a shot on foot as well if the horst doesn't work out straight away. It's challenging in its own right but in a different way that may suit you better.