I was confused about this because my fire does quite a bit of damage at this point, and the wiki says NOTHING about being immune to fire... I've fought Big Blues and Dread Collectors and they're SUCH A PAIN because I love playing Promethean characters and they simply take NO fire damage! Somebody clarify that they're immune to fire or that something else is up, please.
Here's some of Big Red's stats (check Dredmorpedia for the complete stats -- apparently I can't post that many images, and the icons count as images). 30 20-5 10196514 etc. As you can see, fire is its main resistance. I'm not exactly sure how the base stats are altered with difficulty level, but they are, so that 20 could be higher (20 means that the first 20 fire damage is ignored).
Okay, I'll definitely check Dredmorpedia from now on, rather than the wiki. Thanks. I guess that explains it!