so scotia turned into a 3rd monster that cant be harmed. which makes the game unfinishable.. does this mean you have to restart? pretty crappy if true.

It's part of the endgame.

*** SPOILERS BELOW ***

























From the Wikipedia page:
During their quest, they come across Ruby of Truth, and when King Richard is cured, he gives the party the Shard of Truth, which combines with the Ruby to make the Whole Truth. The Whole Truth is then used against Scotia to defeat her.

To combine them, just pick one up and place it on top of the other in the Inventory at the bottom of the screen. You actually should use the Whole Truth on her before her first transformation completes, I believe. This prevents her from transforming and you can then proceed to defeat her.

But I always use it between her second and third transformations, just for fun. The Serpentis is so cool - in fact they used screenshots of the Serpentis in a forest for the back of the game box.

Fun fact: the reason the Executioner is unbeatable is because it has the highest armor stat of any enemy in the game, resulting in all weapons having a 0% hit chance. I'm not happy that they turned the Executioner into a common enemy (and an extremely weak one at that) in Guardians of Destiny.

That's not quite true - while it does have the highest armor in the game, it technically can still be hit, it's just very hard. The real reason it can't be killed is because it literally has no hit points.