'in its worst' is correct and usable in written English.
You can use it when describing an extreme end of a situation or circumstance. For example, "The storm was at its worst when it reached our town."
You hate advertising, and rightly so, because you've been subjected to it in its worst possible guise.
Even in its worst times, it has always had these two faces.
It is now in its worst slide ever.
It's socialism in its worst form.
It seems like a bloody waste of law enforcement resources, and it's pure profiling in its worst form".
World trade in general remains in its worst slump for generations, although it too is no longer falling.
Formspring, in its worst form, legitimizes the idea that it is truly possible to find out what all teenagers want to know — what people think of them.
It is widely agreed the country is in its worst economic bind since 1991.India is not being singled out.
MAGAZINE advertising is in its worst slump for decades.
Now, she said, "Iraq is in its worst time.
The capital may now be in its worst shape ever.
