The question "In this situation, what are the best tactical uses for at-will major creation?" seems complete except for perhaps needing more information about the environment in which the final confrontation will occur. A user experienced with the ins and outs of the major creation spell (and, maybe, the creatures listed in the question) should be able to help the asker with appropriate strategies.
I imagine determining a best answer is possible: a good answer recognizes both the power and limits of the spell then offers specific suggestions to deal with the PCs, their allies, and—perhaps—the primary antagonist's own ex-allies. And an answer that does that in an innovative and outside-the-box way—suitable for the antagonist's massive intelligence—would be exceptional.
If the question needs to offer more information, edit history will show that the asker is willing to provide that. Additional suggestions as to how to get this question reopened are welcome, of course.
Note: To be clear about where I stand, as a DM I've used the creature that the question asks about in a campaign, and the creature is enormously complicated. That the asker's struggling only with this one aspect of the creature and only in this situation seemed to me to make this question sufficiently narrow, especially after the user provided more information so as to make this essentially a monster optimization question. However, after the asker's edit, I thought that readers might find some of that new information overwhelming, so in an attempt to get the question reopened, I edited it. Moments after I did, the question received a commentless downvote for which I now hold myself partially responsible. To be extra clear, I don't already have an answer prepared for this question, and now I'll probably avoid answering it on principle.