Unless I'm reading this wrong, you want a condition that triggers when all objects of a certain type are destroyed, yes?
If so, it's quite simple (but easily missed if you're new to Construct).
Use the System>Compare to check the number of objects, like so:
+ System: Enemy1.Count Equal to 0
and then you can make your "End of Level" sprite appear.
If you're still not sure how to get to that, double-click the Event sheet, double-click the System object, and scroll down to Compare.
Then, highlight value 1 (should be already) and double-click your enemy1 sprite in the bottom pane, and then under Misc, you should see "get number of objects".
Double-click that and then click Finish.