First of all, it's because of bad rolls, for sure. Sounds like a self-excuse, but still it's true.
Secondly, it was because of the wrong positioning. I placed my three characters (the archers and the cleric) in sight of the enemy's wizard, and, you know, he was built to kill.
Third, it was a mistake to leave the paladin without attention. He was standing all the game in the middle and gained a great deal of points. Yes, perhaps, it wouldn't be easy to kill him, but at least I could have tried to fight the middle off.
Fourth, I forgot that the bard can give rerolls to allies as an immediate action. It could have saved my cleric's life and could have negated the ranger's stunned condition. I just didn't remember it.
And fifth, I had chosen the wrong spells. For example, the bard has a spell which increases the DC of his spells by 1, and I needed this 1 when the enemy's Psychic was trying to resist the bard's Hold Person. In place of this I used something useless, I don't even remember what was that.