Alright, I'm investigating it now.
Edit: I didn't get the error going through the fight, unfortunately. I've not had any error reports about it from other players either. Maybe it's something to do with the way the old hotfix patches were applied?
A problem here is that it'll take time to test whether the fix works or not because of the length of the battle. So what we could do is, if you download and apply the recent NT update (should just patch over your current install; doesn't need any hotfixes yet) I'll prepare a special scene hotfix that'll set that battle to be over very quickly; that way you can check to see if the field screen is still malfunctioning or not without having to go through the full fight every time.
Edit 2: So if we give that a try, here's what to do: Download the latest Main Installer and install it; it should patch over the previous NT build with no issues (but let me know if any errors pop up, particularly with the flevel). Once that's in, apply this patch:
https://mega.co.nz/#!P1NT0CbJ!jzo2X7b0XusedUdRybJP9FzCg9_unDiUjwW6q-Yp2_MThat will set the 2nd Brass Dragon form's HP to 286, so you should be able to one-shot him. That'll end the battle much quicker and let you check if the field/script is working properly again. If not, then there's a few work-arounds we could try instead.