Algorithmic approach - find center of circle with given radius covering maximal amount of creatures. I think that such algorithm (if exists) is rather complex (example, another example) and is overkill for this relatively small field size.
Brutforce algorithm - calculate number of creatures inside all the possible circles
Reverse algorithm - requires continuous work during creature moving - keep array of integers - every element is number of creatures attainable from this point. When creature is moving right, decrement left cells, increment right cells and so on. Example for two creatures in (2,3) and (4,3) coordinates. (3,3) point is the best place to bomb.
0 0 1 0 0
0 1 1 1 0
0 0 2 0 1
0 1 1 1 1
0 0 1 1 1
What method is better - depends on some conditions - do creatures (frequently) move? and so on.