A wild spoiler appears! (Do the poke'mon battle music in your head if you want)
*** Spoiler - click to reveal *** So after completing the arena the Gallic brothers tell you that the Crocodile is in fact an old woman called Berenike. Travel to Kerkesoucha Granary and put her out of Egypts misery. After her final cutscene on this mortal coil, the achievement should pop.
Let's keep cheevin' together!