Hi,
two solutions:
-- you turn this in its head and have your ennemies know about this trigger, and so each enemy is responsible for that management. It's more object oriented and more flexible in the long run.
-- you use an FsmArray of type GameObject, and everytime ou have a trigger Enter, you add that GameObject to the list, everytime you get a trigger Exit you remove it from the list. This should be on each triggers.
Does that make sense?
Bye,
Jean