Most efficient way to kill primeval demons/black phantoms in black world tendency?

i keep dying in human form get to PBWT then i kill either demon first or the black phantom whichever close to me then my world tendency increases again, now i need to die 3 more times to spawn the other one, its simply asks for too much item that gives you human form, like if you wanna kill both invader and the demon you need 8-9 items and i simply dont have that much for 5 archstones and is there a trick to this, like killing black phantom or the demon first affects the item usage in any way? for example i killed the primeval demon in 2-1 but then run all the way to the 2-2 without resting all the black phantom miners spawned but scirvir wasnt there, why? Techically im still in PBWT because other dudes spawned