That was not Serena that was like 20% of Serena. And 20% Serena hurt Glidartz. Just look at his scratches. Read the chapter again please.
If 20% of Serena can hurt Glidartz, and 100% Serena can potentially beat Glidartz, I don't even want to know what the man you call butter knife durability can do to him.
@Takuan Yeah but Gajeel dying from this only shows a hax-ish technique from Bradman. I believe characters like Laxus or Gray would also be affected by that "deintegration" that Bradman uses as his last card; I don't necessarily believe it makes DF Gajeel weaker.
As for Gray, as much as I don't want to believe he'll use such a power without being triggered by any such sad event, I think we're naïve to think so; since it's rather clear Mashima's intention was for Gray and Natsu's portrayal to still be rather equal, in the sense that Gray could fight Natsu and still have a shot at winning. Whereas Natsu's feats are above Gray's (in the sense Natsu wasn't bloodlusted and two-shotted a spriggan, whereas Gray had to become bloodlusted to do so), they're still shown as equals. I believe he'll be able to use that power again at will... So he can already be considered a powerhouse on the level of Laxus...
Pre-ice lock Gray, even in DeS mode, is a tier below Invel indeed. You make a lot of sense actually.