Even though the shaman was preparing to attack no one noticed, well it cannot be said no one noticed but it had to be no one in the mood to notice what the shaman is doing after all they are busy in the fight with the ogre and the ogre aren't giving them any chance of moving. And as for the one who shouted is already wounded leaning against the wall.
Shaman didn't give anyone a single second to think as the ball of flame above his head is already thrown toward the blessed with clear intention of wiping out all the blessed is one sweep. The shaman knows very well and believes that he will win no matter what, but does not want to take risks at all.