Hmmm I want the end trigger to activate only when a certain unit (in my case a tank)
I have not understood very well that. You mean that you want the tank to be in the trigger's area??
If it is so, let me see. Create the trigger over the area you want to be without enemies. Put "Activated by East (or west or restiance, it depends on who are your enemies) NOT PRESENT. Now put in the On activation: clear=true
Now another trigger covering the same area, group it (F2) with the tank. Put in the on condition field:
this AND clear
Now that trigger must be and end one. And it is done.