If you want to clean boss rush in 6 minutes you need:

1. Level 40 (can up in boss rush or when farming gems)
   
2. Maximum Time (235, can buy from Nitori)
   
3. Maximum Knives (48, also buy from Nitori)

For fast level up you can clean boss rush, but if you dont have max time&knives, go to extra stage, room right after portal gates.

• For fast restoring MP and time just farm Graze a little (like in 0:47-0:59)
  
• Firstly use thousand daggers without time stop, so that the second snake(or dragon?) gets closer to stop time and secondly use thousand daggers
  
• Diamonds can buff your ATK a little,

If you have completed the preparations, go to boss rush.
As u pass, check with these clauses

• Kill Meiling, Marisa and Patchouli faster than one minute
  
• Get to Reimu faster than ~03:30 minutes

For fast kill Meiling stop time, use thousand daggers, reset time stop and use thousand daggers again.
For Marisa use 2-3 thousand daggers with time stop. After this you can kill her after one or two thousand daggers without time stop.
As for Patchouli, continue spamming thousand daggers, but keep her at the distance to let the knives fly into her and not fly away
Most of her spellcards damage can be ignored when you use thousand daggers (same for any boss), but for faster clean restore MP and time on spellcard, when Remilia launch much red crystals (1:39). The rest of the time, keep your distance and constantly attack. Part with lasers also can be ignored with thousand daggers, but try to kill her before she uses it.
If you have full time, use as much as possible thousand daggers with time stop, and keep using after the time will end. After that Nitori is gonna have very low HP and you can finish her off.
Try to keep close to her and attack with and without stopping time. You can restore MP and time on fireball spellcard (3:08, initial attack), and when she launch shooting spheres (3:34)
You should kill her before she enters the third form.
Not so hard boss, if you know how to kill her right. For first form just spam thousand daggers and restore MP and time when she launch fireballs (go behind her and stop time several times).
in second form the initial attack solves almost everything, if you make a mistake, you have to start over. Get to Reimu faster than she makes cards and use thousand daggers (without stopping time for first), you get so much graze and time, so keep using thousand daggers close to her until your MP and time ended.
Some tips

• You also can ignore her fireballs in first form without stopping time behind her. Get off to a distance and use thousand daggers, so you ignore fireballs and get graze (but last two can hit you)
  
• You can ignore her fireballs in second form if you use thousand daggers.
  
• When she creates seals before the attack with a laser, stop time, destroy some and get off to a safe distance (nice time for 1-2 thousand daggers

My walkthrough of the boss rush

The faster method (55 seconds faster), but there are some issues with a thousand daggers, after fixing them, you can keep within 5 minutes.


My faults:

1:05 - can one more thousand daggers with time stop.
1:18 - use daggers when Remilia blocking it + bad daggers.
Have a so less time on Nitori.
1:55 - really bad use thousand daggers.
1:58 - can one more thousand daggers.
Have a so less time on Flan.
2:33 - nah im so stupid.
3:32 - you see it, yes.
4:15-4:24 - life is a ups and downs. But it seems I am only falling.
4:28-4:34 - omg what am i doing idiot.
4:43 - no words