So, to defeat the tree you need to keep hitting it. Preferably the heavy attack (hold SPACE), dodge the falling creature (Ctrl) and regular attacks (Tap SPACE) on the slime. Alteratively, you can use fast light multiple attacks (FORWARD+TAP then HOLD SPACE) if you are low on HUNGER. You need to rotate, from dodging and killing the slime, to keep the trees mouth open long enough to kill its heart after doing enough damage to its body. Your level/stats will come in to play on how easy the battle will be.
Also, with the most recent, updated version of the game, he made it impossible to get to the other side of the tree like one could do before.