Leo tells Prue if they were meant to defeat Tuatha, there would be a spell in the Book of Shadows and Piper and Phoebe confirm there isn't one. Other times throughout the series, there isn't a spell in the book to defeat someone or something and they write one themselves. They have also added spells to the book. The fact that there isn't a spell in the book shouldn't be indicative of them not being meant to defeat her.
Three times during the show the snake is heard to rattle and hiss, but this is a boa constrictor that does neither. It is obvious when Tuatha picks it up that it it doesn't have rattles. Also the boa constrictor poses as if to strike Betty, but this is totally atypical of a boa.
Tuatha murders the three lost hikers in the woods from the movie "The Blair Witch Project". "Charmed" is set in San Francisco, California. The movie "The Blair Witch Project" is set in Burkittsville, Maryland.
When Dan comes to talk to Piper (before the girls go to find Tuatha the first time) he sees Leo and says 'He's still here?' to which Leo replies 'Never left actually.' His reply makes no sense since Dan didn't ask if he was there AGAIN, but there STILL.