This is something I've been meaning to mention, Bradman was either influenced by PIS/CIS (didn't use his best curses) or he started off casually. My argument for the latter is:
As you can see, Bradman hasn't fought properly in a while and outright states that he'll be fighting to his heart's content. THIS is why he didn't start of with his best curses, it was only towards the middle-end of the fight when he began using his best curse (Over Skelter), at which point Gajeel ate the MBP's. Had the battle gone on normally, Bradman would have killed him as Gajeel wouldn't have eaten the MBP's. And Gajeel was clearly at deaths door just before he ate the MBP's, Bradman sent a thorn through him easily.
Had Gajeel camped in the shadows he would have been an idiot, as we've seen Shadows can hit Bradman. Even then, why would he choose to sit in the shadows when Levy is in the vicinity and Bradman is killing those around him via the First Seal?
Over Skelter is
technically his strongest curse but, as I mentioned earlier, it isn't anything like a nuke, more than anything it's a trap. The "whirlpool" of skulls is so that the opponent can't escape it whilst they're being drained of life via the skulls and MBP's. Basically it damages over time, if the person (however unlikely) can escape early on like Gajeel then it's no surprise that they won't be affected too badly.
As for JBI/DF Gajeel, yes Bradman couldn't react but in the end Bradman can stalemate that by using his suicide attack. Besides, JBI/DF was a
massive (arguably plot induced) power up, he went from being massacred by Bradman to outright blitzing him. But this doesn't matter as he can still stalemate via his suicide attack.
Being honest I only see one reason why DF Gajeel beat Bradman: speed. We know that Dragon Force amps a characters physical stats like crazy, had DF Gajeel gone a bit slower then I'm of the opinion that Bradman could still have won. Had a mage on the
power level of DF Gajeel been up against Bradman then I think the fight would be an extreme-diff fight as it isn't the
power that did Bradman in, it was the sheer speed of the attacks.
Keyes vs Juvia isn't the best example, Juvia had to get inside Keyes and destroy him from the inside, the rest of the time Keyes was easily avoiding her attacks. Nothing is stopping Bradman from just doing what Keyes did or Absorbing the souls of spells.
I disagree on God Serena's attacks being weaker than Gajeel, Gildarts hinted that Goddy would have given him a high-extreme diff fight and given that Gildarts can crash spells fairly easily, his attack power must have been high.
The same Levy who had a mask on during the fight? And even then succumbed to the MBP's shortly after? Levy only lasted that long because the MBP's didn't enter her lungs, had they got to any vital organs she would have been dead before the fight ended.
The problem with people making a mask is that Bradman would likely just attack them as they try to make one, Levy entered the fight with the mask on, if someone tries to put one on mid-battle then Bradman is going to attack them. Unless they have speed that's equal to DF Gajeel, they're getting killed for having no Iron Lungs.
That's assuming the person is as fast as DF Gajeel, if they can't match the speed and eliminate Bradman quickly then he'll start using his haxier curses like Sensation and Enhancement whilst mitigating some damage via Absorption and MBP body. Beating Bradman is all about speed IMO, you can have great power but the second his hax starts getting spammed (especially his most hax curses) you're basically finished. This is obviously excluding people who would one-shot him like August and Irene.
In a battle though, a passive means of killing can be more deadly then a stronger attack as it allows you to attack as the passive killer is doing it's job. It would be a bit like having a gun fight in air polluted with nerve gas whilst not wearing a gas mask.