There are many reasons why the Sorceresses Assassination plan failed or why it was dumb. For starters, our heroes observed that the Sorceress can casually walk through solid material. She demonstrated this power twice right before their eyes. As such, closing her in a gate doesn't work.

You're also forgetting the Deling Abduction Plan, which was similarly full of holes and also failed.

There are probably more examples in the game as well.