I found the Arka I trials are best done in a Cataclysm burn style in Abyssea. I personally used Hell's Guardian, Beyond, and Minikin Monstrosity for my Atmas. Gather a bunch of mobs up and start the Cataclysm. When the first mob if low enough HP, just hit it with a Holy or Holy II for the light kill then move onto the next mob to continue Cata burning. Once they're low enough, hit 'em with a Banishga II to kill 'em all at once. Works very well WHM/RDM and mobbing at least 5 at a time as your TP generation is pretty boss with that many mobs hitting you.
Good luck and happy hunting!