I managed to kill all the pepper rexes needed for the monster eradication goal. I haven't even done all the rock crabs yet, I never had them done this early. I knew I killed some - one rex floor and a few stray ones on the lower levels.
So today I happened to find a pepper rex floor on level 9, with at least six. So after I killed them all I decided to go back up and try again instead of going further down. Just to get that goal out of the way.
So I went out, went back in, and killed another 7. Then the level was a mummy level, so no rexes, but the dive after that the floor had another 5, and apparently I only needed 4. So glad to be done with them!! I never had a floor with so many before, and it surprised me that both repeats were pretty big, too. Sometimes dino floors have one or two and a million green flies...
I still need to kill serpents, rock crabs, slimes and mummies. And those irritating flame spirits on GI. I did find a holy blade, so the mummies shouldn't be too difficult.