Hassan

Currently Hassan of Serenity won't be your best choice for a 3 star Assassin, however after her interlude which is released in 2 more singularities she will gain a large power spike and another one when the 6th wave of strengthening quests are released. The Interlude will make her second ability not only generate crit stars, but inflict a poison as well as doubling the damage of all poison effects on a single target. Meaning with her upgraded skill and NP you are looking at 2500 damage a turn or 12,500 damage total over the 5 turns. Currently her main issue with usability is the fact that without her interlude or strengthening quest, she is just a worst version of Assassin Shiki damage wise, with the benefit of being able to seal skills and NP.

Now as she is waifu status, you are going to want to build her around an arts team, as her main source of damage will be her NP. Meaning for high rarity recommendations you would run her with Waver and Tamamo no Mae. Lower tier recommendations are Hans Christian Anderson because of his QAAAB deck his increase to crit damage which Serenity will want to increase her card damage, as well as his easy to use NP. Finally Mozart, who's first ability will increase arts performance of Serenity, as well as when his strengthening quest comes out in a week or so from now he will be able to drop a ton of crit stars with his new ability. As both Hans and Mozart are casters Serenity will be able to absorb most of the crit stars that get dropped.

For craft essence recommendations for Serenity, Another Ending will be what you want providing arts performance for her np and two art cards as well as crit damage. Formal Craft and Similar CEs are another decent pick to increase arts performance of np as well as to help out her mediocre NP gain. If none of these options are available or you want to just spam your NP more options such as Divine Banquet, Prima Cosmos, and the lower tier Jeweled Sword of Zelretch will be good pickups.

If you do decide to put a lot into her I would recommend leveling skill 3 first to increase the debuff chance of her np to 90% which is Serenity's main niche as well as adding a bit more death chance.

I hope this helped out, and good luck!