Your question isn't very clear, but it appears you want some way to group enemies into clouds to surround them.
One approach is Cluster Analysis. Start with a list of all enemies and a table of the distances among them. Group the two closest enemies into a cluster. The cluster becomes a new "enemy" with a centroid (average X and Y coordinates). Continue grouping enemies until the closest are above some threshold. The clusters of enemies remaining are your compact clouds.
If you have many enemies to group, a KD-tree (http://en.wikipedia.org/wiki/Kd-tree
]) is efficient for finding nearest neighbors.