Flamie said that the barrier, though strong, wasn't the strongest she felt before. She's only half fiend, so what if there's a fiend around, much stronger than flamie and with some brains? Such a fiend could very well enter the barrier and create the situation here.
Regarding Adlet's poor defence, well it's true that he was cornered by the other 3 that are pure strangers to Adlet's group. So it's a bit convenient when they agree and validate each other's opinions/ideas. On the opposite, Adlet's group is very weak, Adlet unable to think for himself, Nachetanya who can't take sides properly, Goldov solving personal issues and Flamie who doesn't care.
All in all, it's getting way too long to solve that situation we do not really care about.