Learn French: Ce casting est le plus difficile que je fasse avec eux. - This casting call is the most difficult that I am doing with them.

The subjunctive fasse is triggered by the superlative phrase le plus difficile (the most difficult). In French, when a superlative is followed by a relative clause starting with que or qui, the verb in that clause usually takes the subjunctive. This shows that the statement is your personal experience or subjective opinion rather than an absolute fact universally true for everyone.

Is casting an English word, and how do we know its gender?

Yes, casting is an English loanword used in French to mean a casting call or audition. Most English loanwords in French are assigned the masculine gender by default, which is why this sentence uses the masculine demonstrative adjective ce and the masculine article le.

Why is it que je fasse and not qui je fasse?

We use que because the casting call is the direct object of the verb faire (it is the thing you are doing). You would only use qui if the noun were the subject doing the action in the second part of the sentence.

Why does the sentence end with avec eux instead of avec ils?