Re: the Defiant, it might've been designed with the purpose of fighting the Borg in mind, but, IIRC, it didn't fare too well in its one encounter with them.
We don't know how long the Defiant was involved in the battle with the Borg Cube. It could have been minutes, hours, or days. It certainly looked impressive in terms of handling punishment in FC.
We can see Starfleet was much more prepared this time for the Borg since they were already beefing up their ships for dealing with the Dominon, the cube was heavily damaged this time compared to the cube from BOBW.