So I used Alfonse in the arena for the first time and did not regret it.

So my build is:

+Def forged Folkvangr
Dual Rally
Moonbow
Brazen Atk/Def
Vantage
Drive buff
Close Def 3

I am currently in Great Summoner tier, but it's unlikely that I'll stay. But his results are great. Taking no damage from Brave Hectors and L!Grima's makes me proud of him.