Catria is extremely good at taking out red units and can take out most blue units too. Try switching her over Effie. Karel is a must for the bonus. So it will be Catria, Eirika, Karel, and Julia. If you have extra feathers you can upgrade Sharena as she don't need to worry about archers unlike Catria.
My team actually is Karel, Julia, Effie and clarine, but I do believe that Maria will do a pod job too! If you look closer, you can see that they fit together perfectly and this team is just performing perfectly at arena in both ways, offensive and defensive