I've tried attacking with bombers/frigates, the problem is the bombers/frigates DIE before they get within range (and when frigates do get within range, they quickly get out of range because of their speed). I need a cheap ship to overwhelm them so that they actually get to the frigates to do damage.
The problem is either appropriate ships have too little health, or are too slow, or are just killed before making a dent in their numbers. I had some luck with fighters for cover + Z shredders when they could build up enough points to multiply, but I didn't have access to those, and I had the choice between fighters, cruisers, bombers and tanks. One of those are expendable when you are short on resources, the others aren't so much.