How does one get to the root of the problem of how to make a homicidal tree leave...or at least go into hibernation for the winter?

Yes, they shot the knife and drove it farther in. This was conveniently arranged by the Tabanga dropping Terry (the heroine) on the ground first, so they could get a clear shot at him, and standing right in front of the quicksand pool, so he could fall in. Although he was impervious to bullets, he was apparently vulnerable to a bullet pushing the knife in farther! Amazing. Uncanny. Dumb!!!

Mortally wounded, he fell into the quicksand and sank. End of movie. Girl is reunited with guy. Big musical flourish. HEAD FOR THE GODDAMN EXIT NOW and put a bag over your head in case you might be seen coming out by someone who knows you and realizes you just paid good money for the world's worst horror movie.