Encore or first time? In any case, get viola to shoot from far away and try for headshots. Let the others take the damage and run around so he has less action time. Keep blocking and this will be over pretty quick
I used Retto, Beat (mandatory), and Viola...all about lvl 16 or so, so I don't think you're underleveled at all. What I did was spread them all out in a triangle so Tuba could only attack one at a time (try to keep him on Retto).
You can control Viola's bow shots but make sure you are relatively facing the target or else she'll start punching instead of bringing up the aiming screen. And aim a little higher than you would expect because the arrows fall as they fly farther.