"To file suit" is a phrasal verb because without the article "suit" is a necessary part of the verb phrase. You can't file photo or complaint, you can only file a photo or a complaint. There are many things you can file, but they need their articles. Among them is "a suit", so it is possible to use the transitive verb "file" with "a suit" just as you can use it with "a complaint".
You can't say "that to file suit be a phrasal verb". It sounds as if you were telling the verb to go and be phrasal.
I suppose that in these examples the problem is the prepositional phrase. "Of plaintiff" simply sounds clumsy. But then again, legalese always sounds clumsy... So, don't worry too much about the fact that in legal terminology the words defendant and plaintiff may be used like names rather than descriptive terms, which then need articles, because in all other contexts you will have to use articles anyway. Only lawyers need to care about plaintiffs and defendants without articles.2. As to "defendant" "plaintiff" etc without a determiner.
(...)
- If the evidence is so strongly in favor of the defendant that a
reasonable jury could never find in favor of the plaintiff, then defendant is entitled
to summary judgment, and plaintiff loses, then and there.
Please notice that in the last example "defendant" and "plaintiff" are first mentioned with the determiner and then without any. All these sentences do not tell us a story of someone's lawsuit but explain to usingeneral legal procedures. So "plaintiff" (without the determiner) does not substitute here for a name of a man who is the plaintiff in the case we are discussing now and we know this man. Do you understand what i am trying to say?![]()