I am not was it a glitch or something, but I kind of deserve it for forgeting it and did for three times.
The situtation was like this. I was fighting Mephisto with two Subcinctuses.
We knew that Mephisto only flee when we defeated him, but his vitality will regenerate a little before he flee.
With this battle, Mephisto also uses a skill called "Binding Contract". Which he will take the life from the Subcinctus to heal himself, so Mephisto will not flee until all the Subcinctuses was defeated first.
As the battle went, the Subcinctuses use the "Temptation", which have a counter attack of "Hell Hath No Fury" (the strongest counter I know of. If you can't defeat it one shot, better left it for that turn.) So only target left was Mephisto. During the turn that Mephisto was the only target, I managed to affect him with the effect of "depair" (prevents resurrection and healing) and "Bleeding" (damage receive for performing any hostile action). Sound cool, but not in this case.
So this happened, when Mephisto's vitality hits zero before defeating the Subcinctuses. He can't use the already casted "Binding Contract" to heal/resurrect himself because of "Depair", but his vitality regenerate a little, but again he can't flee becuase the Subcinctuses still around.
So he will use "Call Flame" and "Sear", which both are hostile action that he will received damage because of "Bleeding". Enough to hit zero, but he can't heal/resurrect then his vitality regenerate a little, but again he can't flee. Then he use the two hostile skills again. Then goes on and on until all my heroes gone and I lose.
Kind of unfair, but interesting way to lose? :P