Building an Arts team.

If you were around for Knk event, use Ryougi Shiki as a second Arts DPS.
Robin Hood also works.
Or you can further support Shuten with a Hans to increase her base attack with his NP and cause some NP chains. That's a ton of Arts cards.
Tamamo (caster) or Waver are other (maybe friend servant) options.

If nothing else, Black Keys CAN give you 10% boost to Arts when maxed out.
With Shuten, NP damage bonus or debuff success would probably be better than just NP gauge per turn, since you're planning on supporting her in an Arts team. Those kinds of teams don't need much help reaching NP 100%.

If you don't have any Arts boosting CE, try critical boosting ones, instead. Fragment of 2030, His Rightful Place, or Golden Catches the Carp all generate tons of stars for Shuten to use, usually while enhancing her supports' abilities to NP to support her. Once Mozart gets his strengthening quest, these will be great for when his crit star skill is on CD.

Hope this helps!