Blows are struck in Initiative order as normal and, unless the player specifies otherwise, are directed against any rank-and-file models involved in the assault. If characters or other models that can normally be singled out if they are in base contact are involved in the assault, then up to half the enemy models can choose to attack them.
Note that casualties are taken from the 'back' of the unit as normal, so all models will get to fight, assuming there are enough left to do so.