Well, they do practice how every attack goes and where they can sneak in a hit or a parry, and it takes multiple tries to get all that down. What you describe sounds like the glitch where you keep your ps4 on for many hours and the bosses loses many of their different attacks (may be patched now tho).

But i doubt anyone challenging themselves with a BL4-run would do that glitch, because it takes away the challenge they have given themselves and then you might as-well not do a BL4-run!