Ryougi Shiki (Assassin)

• Mystic Eyes of Death Perception is a no brainer to max first. This skill is Shiki's main steroid and allows her single target Noble Phantasm to hit like a truck. Its Invulnerability Pierce component also allows her to bypass most bosses who like to use Dodge or Invulnerability such as Marie Antoinette.
• Yin-Yang should be maxed right after. This skill is extremely useful for Shiki's role and can even facilitate 1st turn Noble Phantasm when combined with Craft Essences like Kaleidoscope or maxed limit break Imaginary Element.
• Mind's Eye (Fake) is her main survival tool while also slightly helps her damage output with the Critical Damage buff component. In general, you will most likely use this skill to avoid a Noble Phantasm from the enemies or stalling for time, and the added Critical Damage bonus is just a bonus. If you want to focus more on survivability instead, maxing this skill before Yin-Yang can be considered. 

One of the best welfare Servants in the game and easily the best 4 star Assassin, Shiki is a well-rounded Arts-focused Assassin that focuses on single target damage.Thanks to her good amount of hit counts, NP generation rate, Art-focused deck, and her Yin-Yang skill, Shiki has no problem generating NP bar and can spam her Noble Phantasm endlessly in an Arts team. Her star generation is also not bad either, enough for her to work well on her own or provide support to teammates. With her natural critical damage buffs from Mind's Eye and Independent Action, Shiki can also function as an Arts critical damage dealer. To round it off, Shiki also has an on-demand dodge, making her well-suited for tough fights.

All in all, Shiki is a consistent and powerful boss killer in Arts team, but can work well independently of team support.

Great NP Generation Rate

Ryougi Shiki has an Arts-focused deck with 2 Arts cards and an Arts Noble Phantasm, allowing her to easily do Arts Chains with teammates.

Her NP generation per hit is quite decent as well (0.8 per hit). While her Arts cards only hit 2 times (3 times for her NP), her Quick cards hit for 4 times each, meaning she can get good NP generation out of her Quick cards and has decent NP refund rate.

Her Yin-Yang skill allows her to instantly get 30% NP bar at level 10, allowing her to use her Noble Phantasm on her 1st turn when paired with Craft Essences like Kaleidoscope or maxed limit break Imaginary Element.

Decent Star Generation Rate

Being an Assassin, Shiki has high base star generation rate by default (25%).

This is further helped by her Quick cards hitting 4 times and having a C rank Presence Concealment.

She is a bit weaker than a similar Assassin, Mysterious Heroine X, due to MHX having a higher star generation passive and a high hit count Quick Noble Phantasm, but it is pretty close in terms of overall output.

Can Function as an Arts Critical Damage Dealer

Shiki is one of the rare Assassins that have critical damage buffs in her skillset, allowing her to serve as a critical damage dealer.

Her low star absorption rate might be a problem, but she can offset it by generating critical stars on her own.

Has an On-Demand Dodge

Mind's Eye allows her to survive a lot more proactively compared to Assassins without it like MHX or Camilla.

Great Single-Target Noble Phantasm

Shiki's Noble Phantasm hits quite hard thanks to her Mystic Eyes of Death Perception skill and high NP level.

It also ignores Defense buffs, and combined with Mystic Eyes of Death Perception's ability to bypass dodge / invulnerability she is a great Servant to bring when faced bosses that has strong defense buff / dodge.

It can also instant kill most mobs due to its secondary effect. However, most bosses will have very low death rate and you should not count on this when using her.

Low Damage with her Normal Cards

Outside of her Noble Phantasm and the occasional critical hits, Shiki has low damage on her normal cards.

This is due to the Assassin's modifier, as well as her lack of Buster cards and long-lasting self-buffs.

One can solve this by putting her into a heavy Arts team with strong buffers to allow her to spam her Noble Phantasm, or/and supply her with a lot of critical stars.

Poor Durability over Time

Assassin Shiki's HP pool is quite low for a 4 star Servant, coupled with the demerit on her Yin-Yang means she might fall down very fast when focused, especially when her dodge is on cooldown.

It is best to pair her with someone that also improve her survivability, such as Jeanne d'Arc or Tamamo no Mae.

• Formalcraft / Projection: With an Arts-focused deck, more Arts performance booster is never a bad thing. Assassin Shiki gains a lot from these Craft Essences in term of both damage output and NP generation rate.
• Halloween Princess: Getting a solid NP damage buff will vastly increase her NP damage output, while the starting NP gauge lets her pop off her first NP much sooner. Thanks to Yin-Yang she will also get to 100% NP bar easily.
• Little Halloween Devil / Divine Banquet: Further improving her NP generation rate is not a bad idea, and Little Halloween Devil can also helps her use her 1st NP much faster when you need to burst the boss down fast.
• Heaven's Feel / Black Grail: Her Noble Phantasm hits really hard and Heaven's Feel will make it even more devastating. You can consider Black Grail as well but with her Yin-Yang and Black Grail's demerit she can fall quite fast.
• Kitchen Patissiere: A solid choice for Shiki as she needs both the star generation and NP generation buff, making her all around better.
• Another Ending: A really strong Craft Essence for her if the team can help her with star generation (such as having Nero Bride or Fragments of 2030) to facilitate her critical attacks.