Not to be confused with the more 'leggy' Treant with arms seen in other tales games.
This boss has got a wider range of attacks than it's smaller counterparts, can cast some artes, and will start to annoy you with unblockable jumps near the end of its HP. Luckily it's weak to basic attacks, so chaining combos with your BG can cause a ton of damage. I found the best way to effectively defeat the boss is to continue to use BG in chains, and wait for the AI to almost finish their attacks to help continue chains. Also, if you change to Rose when Sorey's BG is out, you can continue to use the BG to continue with Rose's basic attacks and just keep spamming for a long time. The tree is also not resistant to Fire, so Lialah is great in this battle for staggering potential. If you can keep the combos going with BG, you'll easily defeat the tree no problem.