I would count the swarm as one unit, and for every point of damage, then one member of the swarm is killed. As for the attack power, you could just have it stay the same every time, regardless of how many members of the swarm is left. Or, if you want it to get complicated... have their attack be 1D6 if there are 6 swarm members. For every member over 6 of them, add one to the roll. If there are less than 6 members, than subtract one from the roll for every member under six.
For example, lets say my wizard encounters a swarm of enchanted skulls. (Ten of them)Being the awesome hero he is, hed probably just cast a spell to kill them all with a giant fireball, but for the purpose of this example, lets just say he will hit them with his staff, killing them melee style. If he attacked them, and made his skill roll, and got a 10, they rolled defense and got a 4, and add four because there are 10 of them. My hero would kill two of them. They attack him, rolling 1D6+2 (Because there are eight of them.)