I think the best way to do this is by making 2 triggers. Give the first the event "29 Destroyed by anything [not infiltrate]", which will activate the trigger when the attached object is destroyed (not infiltrated). So attach the tag of this building to the building you want to trigger it when it's destroyed.
This is probably what you did, but where I think you went wrong is, you probably also added the action to disable/destroy the buildings to this trigger and also attached the buildings you wanted to be destroyed. This means that all buildings attached to it will both trigger the event of this trigger and get disabled/destroyed by the action of this trigger.
So what you do is, go to the Edit menu and go to 'Local variables (Locals)'. Now type something for the description; just call it "boom" or whatever and pick Preset value 0 (clear).
Now go back to that trigger you was working on and give it the action type "56 Local Set..." and pick for the parameter value "0 boom,0".
Now make another trigger, which has the action to disable/destroy the attached buildings, which should obviously be attached to the obelisks (and whatever other buildings you want to attach it to). Give this trigger the event "36 Local is set..." and pick for the parameter value "0 boom,0".
I haven't tried this, but I think it should work. So, good luck