For farming, Jiang Ziya is mostly relegated to battery farm setups. With Kaleid and proper supports, Jiang Ziya can loop through batteries alone. Odds are you have better farmers, though—even Habetrot is likely a better option, assuming you’ve cleared LB6—so I won’t be devoting a whole lot of attention to farming setups beyond “they exist, though they mostly need Kaleid.”
Jiang Ziya is much more interesting in CQ contexts. As a DPS, he’s remarkably self-sufficient. He brings high steroids and a selfish battery by default, which means anything else you can give him is bonus. Any standard Quick setups will work well here—Skadi/Skadi/Oberon for blitzing, Skadi/Castoria for stall, etc. Skadi/Jiang Ziya/Castoria is a particularly good team, as Jiang Ziya’s battery helps the other two to use their NPs more reliably. You can also slot in any of the alternative Quick supports or semi-supports if you’re missing meta Support Servants or are having cost issues. Barti gets a special shoutout here, as Barti can push Jiang Ziya’s crit damage (which he’s missing otherwise) while also being able to leverage Jiang Ziya’s party buffs for good damage output himself, especially versus Casters.
Jiang Ziya’s supportive value is more questionable, but he can function in that role too. Given his long cooldowns and lack of survival tools, he really wants to be supporting fast-moving Quick teams. He’ll work best as a third-support for double Skadi teams, especially for players who missed Oberon and Santa Martha. You can also run Jiang Ziya/DPS/Skadi, or even Jiang Ziya/DPS/Jiang Ziya. In these teams, Jiang Ziya functions as a semi-support, outputting good damage himself while also propping up the DPS.
As a damage-dealer, Jiang Ziya benefits most from Black Grail, as always. Traces of Christmas is also a strong option, letting Jiang Ziya NP immediately with just his own batteries and boosting his effective NP gain. In supportive contexts, Devilish Bodhisattva is a strong pick, as it lets him NP immediately and increases the buff value of his NP. Poster Girl is also an option if you don’t want to worry about his long cooldown times—Jiang Ziya can take hits for a few turns before dying to make way for someone more sustain-oriented.
Jiang Ziya has decent crit access due to his star gen and Rider star weight, but he lacks crit damage buffs. As such, crit up CCs are a good pick here. You can also give Jiang Ziya anti-divine CCs to increase his firepower versus divine enemies, especially if you only plan to bring him against his niche.