The higher you get the more preferrable Str becomes over Int for Champs. Asura is pretty much Atk multiplied by SP/8 plus 1000 bonus damage. As SP goes up automatically, your multiplicant does too. But the base number, Atk, will only go up by increasing Str.
Ergo, by increasing Str there is more Atk to be multiplied by your standardly high SP. You'd be wise to still put points into it after maxing Str and getting insta-cast, though.
So, as said, away with Agi and more Int.