My Arena team's weaknesses are pretty dependent on week. My core at the moment is basically Virion 4* + 9 and SS Nino 5* + 10: Nino can safely 1v1 all non-gimmick green/blue units (She needs defense tiles to successfully tank and kill a BIke DC Bonfire build), while Virion, who has slightly higher HP than Nino, is responsible for reloading Nino's HP bar and killing non-firesweep Bow Lyns (though brave bow builds require me to whip out my calculator). I also use a poorly built Lancina (has Renewal 3 because that was the only 240 SP skill I could spare for scoring) for weakly checking reds.
This leaves one slot open for the last unit:
Alfonse weeks (none have happened since this team setup) should be alright, since he has swordbreaker. The Tikis would still be slightly problematic, but Nino can 1RKO them more often than not.
Anna weeks are terrible; I have her decked out with a generic Fury/Desperation build, but she literally just sits on the backline and gets mosquito'd on by Virion.
Funnily enough, I don't use Sharena on her week even though she would theoretically be the most synergistic with my team, and usually just run Olivia (4* + 8, which seems to score better than Sharena does) + Virion + Lancina + Nino, since Virion and Olivia give full coverage over Sharena weeks. Playing Arena with a dancer more or less grants free wins, so obviously this rotation is easiest for me.