Best Arena Alfonse build ?

Side note I would recommend that you merge up units to use as a core team as they will provide you with the maximum score. You can read more about it on the website for more details. Any unit can work for your arena core they should just be available at 3-4 star. https://fireemblem.gamepress.gg/arena-basics

As for Alfonse most people stack brazens on him. Brazen Def/Res works and so does Atk/Def if you want. Refine his weapon for Res. B skill can be QR or Vantage typically. Guard works also. C skill can be any buff/smoke. Special typically Glimmer for budget and Aether for scoring.