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, champions, 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 (as often such heroes will be performing derring-do in the thick of fighting, at doorways or atop stairwells).
Note that casualties are taken from the 'back' of the unit as normal. If one side suffers casualties before it fights, extra models are assumed to step up and join the assault party in place of fallen comrades, assuming there are enough survivors to do so. So ten models will always get to fight, assuming there are enough left to do so!