... that depends if you talk about the anime or at the end of the web novel

End of w.n Rimuru stomps

First of all, this fight would never happen, Milim is too nice, second of all Rimuru would probably body double to make the fight unfair. After all, Rimuru can fight dirty

Currently in the manga, anime and light novel milim would win but if you take into consideration the web novel rimuru would be the winner