This reminds me the Set Cover Problem:
You have a set U that stores the N monsters: U = {1, 2, ..., N}, a set S of possible attacks. Each attack is as well a set of the indexes of the monster that the attack kills. In your example, S is S = { {1}, {2}, {}, {1}, {2} }. You have to find the smallest set C in S whose union is U.
