You probably don't mean La trinity, 'cause there aren't bridges on Malden
It is simple on a condition line write:
not (alive x)
You probably new this
Well on the place of x you write the unit name. If you use island objects (I'm not sure of this, I read this from FDF website long ago...)
write the object ID like: not (alive 30375)
or then you write object+ID: (alive object3425)
or something like that...
You can see ID's by checking the box "Show ID's" in the editor. The ID's change everytime you change the objects on the island, so, it must be finished before you can make ID based missions