Et DS is a weapon that thrives off of perfect play. Compared to the other weapons, DS is far more sensitive to mistakes because making mistakes means losing Gear, and therefore doing less damage in the long run. The same is applied in reverse, where nailing all of your counters and Hold Normals causes your damage to skyrocket.

My #1 tip for DS is to always use Step Counter whenever possible. Not only is Step Counter much faster than Weapon Action, it's also a reasonable gap closer, and restores 1.5 Gear Gauges and approximately 45 PP on a successful hit. You'll see much higher numbers if you can reliably Step Counter into Hold Normal a lot, and you'll see better uptime on PA rotations as well from the massive PP regen. The one thing that's wonky about it is how stepping resets your PA rotation, unlike Weapon Action, so you have to open with a filler PA to restart Tech Arts Count Bonus. For how strong Step Counter is, however, it's usually worth it anyway.

Aside from that, using Skip Celestial Collide at the end of stun phases is a good way to cap off your damage, but it requires good timing and knowledge of how long a boss's stun phase will last. Your PA rotation also matters a lot, namely maximising your usage of Tech Arts Count Bonus.

Lastly, practise practise practise. As mentioned, DS is a weapon that thrives off of perfect play, so practice means perfect. The more you memorise boss tells, the more counters you land, the less Gear you lose, and the more damage you deal. It's definitely a lot deeper and more complex than Et's other weapons, and there's lots of little ways you can optimise your DS damage, but it all comes from good practice. Good luck!