What I'm saying is that resin is strong, but it should not be strong enough to resist anything that breaks a tree.
I'm disagreeing with the episode.
However, the issue isn't actually that a resin composite shield stopped a spear. That's perfectly believable.
What's unbelievable is that something that shatters a tree is blocked by said shield. That said.. take a step back. A spear shattering a tree is the real culprit. How does it even do that?
At that point, my brain stopped and accepted the fact that this is anime, people get slammed against rocks, rocks break but their spines don't.. and that's that.