I finally found a way through it!
The reason why the game bugged (I'm only talking about my case) is:

After getting the name of the demon " RAKAZEL" , a voice tells you that you have to finish all the quests left in the broken valley because you will not be able to go back there after killing laiken. thats why i left sassan in the tower and went back to finish everything. when i came back (way after) to finish and summon the demon, the game bugged. so i have reloaded a previous save point (before waking sassan fantom and giving her the ring) Then I did all the side quests left in the broken valley before waking sassan and getting her the ring. Immediately after that, i went to the battle tower with her and fought the soldiers till getting to the point where she summons the demon RAKAZEL without interruption and without doing any side quest. And it worked!
To make sure it's not just coincidence, i reloaded another save point, before choosing the servants (alchemist, necromancer, trainer, enchanter) and did all side quests before meeting sassan, and it worked again.
So what do you think? did you try this?