My arena MvP is a four star unit at the moment, Cordelia.
Lots of people have five star lords, which tend to be red Sword users. The fact that she's a blue lancer, has a brave lance with high speed so hits at least twice first, can fly, has the skill 'pass', plus 'luna' for extra damage now and then, means that she can lurk on terrain and is hard to avoid. She's assassinating Marth, Roy, Lyn, Lucina, Chrom and various others all over the place. Even Takumi only wins if he attacks first- he doesn't even get his close range counter as she hits him twice first.
Really enjoying this now. Initial focus on five star units has given way to effective team building, and I should be getting a healthy amount of feathers from arena attack and defence as I'm using Cordelia as the lead icon, so everyone wants to fight me because the team looks weak. Can't wait to get her upgraded to five star, I've got enough of the badges needed from the training tower, just need to keep playing arena each day for feathers for a few weeks.