This trophy really isn't that hard to do at all and if you've been leveling and grinding ability points, you shouldn't have much of an issue.
I did this at level 54 with the following equipment:
> Asura's Rod
> Holy Miter
> Egoist's Armlet
> White Robe
> Ribbon
And with the following abilities enabled:
> Boost
> Auto-Potion (although it really wasn't needed)
> Auto-Regen
> Auto-Haste
> Clear Headed (just to be on the safe side)
> Antibody (just to be on the safe side)
Fight actually starts at 1:50, before is character setup.
Behemoth's death is around 7:00.
You basically just want to spam Bahamut and let the defense abilities trigger when needed. After a couple of turns, Behemoth dies and you get the trophy. And you get a Circlet, which is a great head piece of Dagger.