What Went Wrong?
It was very easy to incite them when they are afraid of the weird demon and when the argument to attack is based on sound logic. If all of them cannot kill one mid-rank demon, then they do not have any future anymore in the domain of high-rank demons.
The weird demon's intuition became muddled up because of the two pronged attacks and the numerous other ones at the border that she wouldn't be using. In a sense, he was surrounded by danger. So he walked right into her ambush. He had no choice in the matter, it was the only possible outcome. One way or the other she would have gotten him.
Each ambush team had lifted boulders up into the sky and used a rare treasure to freeze them there. The rocks were to create a heavy attack that is purely physical and will force him to use one of his trump cards. It has been shown that if the attack is physical, he can't copy it or resist it if it is strong. His inability to fly means he won't be able to escape the attack.