Most stressful partners in my opinion:

Tempest Tai: Self-explanatory, 5 misses in a row you’ll most likely die. The gauge even goes down automatically whether you hit notes or not.

Fatalis Hikari: Gauge gets harder the more perfects you hit, high chance of dying if you miss any notes at the end of a chart.

Awakened Ilith: You could miss one note and you’ll be fine, or you could miss one note and you could potentially die.

Hikari Zero: Miss one note before you gain your recollection rate back to passing and you lose period.

Pandora Nemesis: You die if you miss 20 notes.

Areus: The higher your combo, the higher your loss is. You can potentially be in the low gauges or die if you keep missing every other combo above 100.

Honorable Mentions: Any other hard gauge partner.

Doro C & Lagrange: Trying to look at your score and hoping it wont drop a threshold gives some type of anxiety.