You are comparing end of story Rimuru with where we are at the moment Rimuru.
As it is now Rimuru didn't even named US Raphael yet.
Another thing is that what you wrote, even being charitable and giving that there is no problem in using Rimuru's status at the end of the story for this discussion, still do not address Milim's ability, especially becuse Milim did not use her Ultimate Skill in the fight with Guy Crisom because she not only can't control it on her own she also did not wanted to kill Guy. Maybe you should pay more attention when reading the story...