I can think of a few ways.
First, if we just want a real straight forward no variable way of doing it, we can do a when-argument using the number-of operator.
This will activate as soon as 4 selected turrets on the Fenris have been destroyed. You could turn that into a directive if you like.
Another, slightly more complex method to show the player the number of turrets remaining would go something like...
edit: D'oh, this should also repeat with the trigger set to the number of turrets. I also forgot the invalidate-argument here! There should be an invalidate-argument <argument> at the end!
Using the directive is variable sexp allows you to put your own number in the directive. So here is would read "Destroy Half Fenris' Turrets [8]". You could also tweak how the variable is calculated to just show how many left to destroy (start the var at 4 and have change that condition to = fenrisTurrets 0).
(More information about conditional arguments can be found here!)
