Oh, that's quite the fight. The bandit boss was quite tough, so I wonder if he was using enhancement magic at a very basic level.
It seems Ren really hasn't practiced wood manipulation that much, so as expected, moving with wooden limbs was slow, whereas when he just had wood armour and was moving by using his muscles and enhancement, he could be fast.
I wonder if Ren's feeling unwell because he's using several types of magic at the same time - wood manipulation and enhancement.
So is the guy who betrayed the boss an adventurer who had infiltrated their group, or just another bandit seeking to take over the group? I guess we'll learn next chapter. He also had a realisation before he went stealthy, but it doesn't seem to have been any insight on the wood monster they were fighting, but rather just that he would have an opportunity to take down the bandit boss. I think he may have noticed the wood monster wasn't killing any of the bandits though and that's definitely a strange thing from a monster being attacked with the intent to kill.
Let's hope that bandit is actually friendly because Ren is hardly in a condition to keep fighting. He needs to split really, although that means the bandits might escape.