i personally think drain atk is great for free player. esp arana, and she's a great tank with b speed =)
i also heard incinerate, also good do a pretty good damage every turn. :O but i never tested my self. (Allies said so tho, and 3-4 my allies)
defense order!!! It raises your def! quite much~~!!!! However I think 6-7* Units of this skill cost a lot, so maybe try rin or corda?
i tried stone breath i think it's not bad too.. however they're all d speed..
but then I would rather get tank+ decent speed and atk instead of concerning too much abt arena skill