The effect of the master level composite toxin on the experimental sample can be summarised in one sentence:
"Whenever a composite toxin lower than the expert level is cleared, a new trainee level composite toxin is randomly generated."
There are 30 composite toxins on the experimental sample that are lower than the expert-level.
Every time Xia Zhou uses an antidote to clear one of them, the master-level composite toxin would transform a small part of itself into a new trainee-level composite toxin, until the master-level composite toxin is entirely consumed.
To clear all toxins at once with a compound antidote, the antidote for the master-level composite toxin must take effect first.
Unfortunately...
After spending several days creating the antidote for the master-level composite toxin, Xia Zhou discovered...
This antidote would react with other toxins on the experimental sample.
While clearing the master-level composite toxin, it would also clear other composite toxins.