So regarding Big Boy, the main script uses <MOV, <FL+ and <FLJ which does make it function smoothly though it only seems to work once. So I'm going to try and replace them with <TRA instead and see what the outcome is.
UPDATE 1: So defeating Big Boy with the new script simply causes the boss to regain their HP, leaving the fight in an endless loop.
UPDATE 2: So in order for Heavy Press's death animation to work, it has to #1000 or other thousands.