Basically, amongst other arguments, I've pointed out the low levels of destruction caused by the Breen in Cfoe. My opponent is arguing the damage could be due to shield bleedthrough, or even due to an away team.
Breen in what? Cfoe?
A similar argument exists about the Borg bombardment of Cochrane's missile complex in FC. My argument is that the Borg would not have repeatedly fired at the surface if they weren't trying to destroy something. He's saying we shouldn't assume anything as for all we know the Borg were trying to instill fear in the local population.
The borg bombardment being weaksauce might possibly due to component failure on the Sphere's part. The cube was blowing up around them as they evac'ed outta there, and when the Enterprise came through the time thing they blew them up with barely any effort at all (4 quantum torpedoes IIRC), which might suggest that the Sphere was already heavily damaged and didn't have its defences/weaponry at full power. The fact that as soon as the Enterprise arrived the Queen beamed the fuck off the Sphere with however many drones she had on hand also lends credence to this.
Your friend's argument is a bit naff though, I'll give you that.
He also appears to want us to disregard all evidence that contradicts the apparent firepower of TDIC, because if TDIC shows high firepower every other example must be flawed somehow.
That doesn't mean you dismiss it. Furthermore a number of factors exist that explain why it should be considered: first, the fleet had as its mission the planetary destruction of the Founder's homeworld in mind, so it stands to reason that they would equip their fleet with ordnance to accomplish that end. Second, trek firepower is explicitly mentioned to have variable yields and power settings (phasers can be set to 1/100th power for wargames exercises and torpedoes can be set to low yields which don't do much appreciable damage - 'The Ultimate Computer' and 'Redemption part two') so there is literally no reason why you have to have one or the other.