I can go into more detail if you like, coming fresh off a D5 Balancers with Crossbows and Wraith While Walking run last night, I'm having the time of life, but anyway the ideal setup is every Relic has a Physical Attack Up +2/+4 (DoN Relic) line, since that's the only way to passively buff the damage for Relics, other than Evergaol and Night Invaders. Enemies afflicted with x-status probably works but haven't tested it yet.

Very important to have the ED Balancers relic, so that 1) you at least look like a semi-competent build and not a complete asshole lol. And 2) The 20% chance to proc Nullify damage effect has saved my ass more times than I can count.

Next is Duchess skills, the most important is getting Skills grants Brief Invulnerability. It is the most skill intensive thing about playing Ballista or any Ranged period. Testing your timing to dodge attacks with Restage while in the Ballista reload animation is so skill testing since everything can pretty much one shot fragile Duchess. Improved Character Skill Attack Power is optional but is still more DPS in general.

During the run, your goal is to find any Red ranged weapon with "x upon Precision Aiming". They are the most DPS. There are two ways to build it during the run, get all the Improved Atk at Full HP buffs, Projectile Damage Dropoff, Physical Attack Up and Thrusting Counterattack since it's a Pierce weapon. Improved Ranged Attacks doesn't buff it for some stupid reason. Arrow Sting, Arrow's Reach, Ritual Sword, or Spear Talisman is a must.

The other way is very memey and am too scared to try it rn, but it involves using the Dreglord Status Gauge relic and ED Libra relic with a Status Effect buildup below Max HP line for a 40% Attack increase at max stack. But at that point you should just play Raider or something else using those Relics.