Dissonant Sonata is a fairly competent offensive ability that has Deuce damage surrounding enemies and possibly inflicting Stun, so she won’t be completely defenseless in a fight. Requiem of Ruin is another ability that allows her to deal damage to surrounding enemies but honestly, Dissonant Sonata is much more effective. You should also upgrade her melee attacks because you’ll burn through her AG pretty quickly by using her abilities and the only way to recover AG is by landing regular hits on enemies.
The thing to note about her offensive abilities is that they’re fairly slow and not very efficient in taking down enemies. You probably won’t be trying to hit Kill Sights and Break Sights with Deuce, but it’s always safer to have an offensive ability on her during the early levels so that she won’t go down so quickly. A good setup for early users is to fill her command slots with Hymn of Healing and either Requiem of Ruin or Dissonant Sonata, and set Wall as her defensive spell.
An ability that you should definitely purchase for Deuce is Crescendo. Think of this as a more effective version of Concerto. Instead of forcing your allies to fight only using physical attacks, Crescendo boosts the attack stats of your allies. This, coupled with Hymn of Healing, is probably her most useful support ability as it greatly increases your squad’s damage output. Upgrading Crescendo will only further amplify that effect. With Deuce, it’s important to keep her HP high at all times and make sure she’s always a safe distance away from enemies.