Cleansing Brilliance is probably the most powerful single target ability for the Advent. However, when fleets start getting large caps find themselves outgunned pretty quick. The AI doesn't usually focus fire but if they have a lot of one type of ship, especially LRF or bombers, your caps won't last long without a lot of support (I.e. a fully developed battle ball).
If you don't think that's it, post a screenshot where we can see both fleets in the Empire tree.
