Shenhe is a cyro support and a sub DPS she scales off of her attack. She works fairly well with Ganyu, Venti Diona, Sucrose, Chongyun, and most cyro based characters. Shenhe gives a good buff to cyro units and makes them do more cyro damage. A good polearm for her is the Calamity Queller and most 5 star polearms such as Vortex Vanquisher, Engulfing Lighting, Skyward spine, and Primordial Jade Winged Spear. Some of the best 4-star weapons for her are Favonious lance, The Catch, and Wavebreakers Fin.
For a sub-DPS build focus entirely on the attack, it’s gonna help her a lot with her cyro damage bonus. For her artifacts, the best ones for sub-DPS are half Gladiator’s Finale and half Shimenawa’s Reminiscence. For the secondary stats use energy rate and cyro damage bonuses also crit dmg and ATK bonuses would work really well.