Phase 1:
This phase is for farming Reiryoku, you probably don't want to spend any unless you need to deal with City of Devouring Time, even then you can consider taking the hit and conserving. You want to go to the maximum 6 turns, and hopefully you can farm 50+ Reiryoku.
The only time Kurumi can get bullets I-IV is turn 1, otherwise she will either use Hei or Tet followed by a generic move. If you get Minute Hand on the turn before phase transition, try to clear the debuff if you can.

Phase 2:
A lot more moves open up and you engage with the main mechanic of swiftness switching. The priority she has for selecting special bullets is: Yud Bet -> Het -> Yud Aleph -> Yud -> Zayin. If she doesn't have enough, then she selects from the special bullet random selection (I-V, IX), except for Aleph and Bet, no same special bullet can appear twice in the same turn. Yud cannot appear 2 turns in a row, unless you hard taunt Kurumi. Spacequake now appears in the generic random selection. Het cannot appear 2 turns in a row, and it will only be chosen if the sum of the clones' health falls below a certain threshold.

Try to keep the use of Reiryoku low if possible, as phase 3 is harder than 1 and 2 put together. And if possible try to break concentration each turn, giving her 3 less Reiryoku to work around is very good. In addition, try not to cast too many swift skills.

Phase 3:
Rasiel will put all of your skills into the deck, note that this interaction means that if you have created skills that don't inherently have exclude, they will lose exclude and you will have them permanently for the rest of the fight (so stuff like medical booklet, sonic blow, combined attack, etc), so try to cast them as to not bloat the deck. If you have a way to copy a good skill, you might want to keep it tho.

This is the hardest phase by far, you now passively gain 7 Reiryoku per turn with your angels but still try to not cast swift skills as giving Kurumi 2 per skill is very dangerous (also you will need to spend a lot on your angels to keep up), also breaking concentration is even more important as it now denies Kurumi 5 instead of 3 Reiryoku. Zayin and Yud can now appear in the random special bullet queue, but if she has the Reiryoku to spend on them then Kurumi can guarantee those attacks. The 2 turn except hard taunt restriction still applies to Yud, and Zayin is guaranteed to appear at least every 3 turns. Rasiel: Future Entry can now appear in the generic random selection.

Vav can only appear if you deal 18% or more of her health in 1 turn, otherwise it will never appear, try not to trigger it.

Strategies:
Read over the Angels and choose which one best suits your team.
Lian can parry Zayin and Yud Aleph (hard taunt doesn't work on them), so you can intentionally give Kurumi enough to spend on that, and then have Lian take the hit. You can still parry even with Yud Aleph (though your skills will still do nothing and be sent into the future).
If you spend nothing but healing and need to stabilize, you can intentionally give Kurumi enough for Yud Bet, it will mean she will heal for a couple hundred health, but since it has the highest priority, it's a good way to avoid Het if you have trouble dealing with the clones she summons.
A good angel that works for most is Camael, having ways to deal with clones, and providing additional healing and debuff removal, if you are struggling to heal the damage she deals. Michael can allow you to "Stun" attacks that normally cannot be stunned, a good emergency button, however she requires a lot of Reiryoku to fuel all of the stuns.

Kurumi has 3 difficulties, 0, 1, 2. The intended is difficulty 1 with base game characters, but you can try 0 or 2 if you wish for a easier or harder run (2 is much harder, not intended for base game characters, neither difficulty is the intended experience). The other option is to change which music plays during phase 3.

The fight is intended to be long, where you use sustain team compositions to stabilize to a safe position and react to the boss moves while slowly and steadily dealing damage. Due to the boss's defensive mechanics, it is impossible to race her and thinking of the strategy of "kill the enemy before the enemy kills you" will not work, expect the average fight to take 20 turns.

Update: added "Speedrun Mode" option to mod options. Enabling this option means you do not gain any Angels, instead you choose any 2 of Kurumi's I-XII bullets to seal. Sealing bullets means she cannot select to cast them so you won't have to worry about facing these bullets, with the exception of phase transition moves, these moves are fixed.