Overview
64 Shiki is an AR that has less of a focus on primarily dealing damage on her own and is more for supporting the rest of the team for players that want to take the extra step for complex but incredibly effective setups. Even without using her gimmick she generally performs about as well as equal rarity ARs like G41, so 64 Shiki is a good option for the commander that wants to go the extra step.
Pros
High base FP
64 Shiki’s firepower of 67 is incredibly high for an AR
Great tiles
High damage and evasion tiles given to both SMGs from position 7 is highly appreciated
Situational and potentially massive team buffs
Can provide an HP shield or an allied firepower buff larger than most in the game.
Cons
Low base ROF
63 rate of fire is on the lower end for an AR
Difficult to predict skill results
Skill is based on number of enemies, which varies wildly
Average self-buff uptime
Self-buff is equivalent to an 8/16 uptime
Long cooldowns and awkward positioning on team buffs
Requires specific positioning of dolls at 8 seconds to make the most out of 64 Shiki
Full analysis
While intimidating and complex, 64 Shiki’s skill can be divided into 2 portions.
Upon skill activation she checks the total number of enemy groups still alive. Linked enemies count as one group, so 3 vespids in 5 links would count as 3 groups. At greater than 5 groups, her self-buff is rate of fire and accuracy, and at lower than 5 groups it is firepower.
While she has decent enough stats on her own, this buff usually has too short a duration to really matter. Since it starts at 5 seconds, only the last 2 seconds stack with common 6 second initial cooldown skills. Because enemy compositions and numbers can vary wildly from battle to battle just from rng or initial positions, it is often hard to control which version 64 Shiki will end up using. This can be an issue since she can easily over-cap rate of fire with the first version or start overkilling targets with too much firepower with the second.
Afterwards the second portion of her skill kicks in. 3 seconds after her skill activation, or 8 seconds from battle start, 64 Shiki does another check, this time for 2 groups of enemies. Above that number and she grants any allies on her tiles an HP shield. Below it and she grants the allies on her tiles one of the largest firepower buffs in the game. Both last 5 seconds, so 8 seconds after skill activation. The 25 HP shield she can grant is incredibly small and usually comes out far too late at 8 seconds to really make an impact on survivability.
However, the firepower buff is where 64 Shiki truly shines. The firepower buff is an incredible 55% to allies and herself. For perspective, the Fire Command skill from Grizzly is only 25%. Additionally, by positioning a doll between both of her frontal tiles when her second skill activation happens, you can stack the buff to get a whopping 140.25% damage buff. This is where one can get creative. By doing this with your dps dolls, you have 5 seconds of ridiculous amounts of burst damage. This can synergize with other burst DPS dolls who specialize in dealing a massive amount of damage in a small window.
There are some issues however. The only time this could happen is with 1 or 2 groups of enemies left on the field, and the only time you would need this much burst for just 1 or 2 enemies is with bosses and incredibly tanky enemies like Uhlans. These kinds of enemies often have mechanics which have you moving your dolls all over the place to avoid damage, making setups difficult.
Additionally, assuming you have 64 Shiki on the back column, this would require you to halt your dps for the moment while you move your DPS to the middle column. You might either risk exposing your dps to enemy incoming fire or move your tanks to the frontmost column, where they have no room to kite.
However, if you manage to pull it off in the right situation, you will be greatly rewarded. Of course you could always just leave her alone and she would perform close to a normal AR like G41, although often weaker because of a low base rate of fire leading to lower ability to fight mobs and the HP shield hardly contributing