Being that he was just recently transformed into a Demon Lord, Rimuru was initially perceived to be weaker than Clayman, who was undoubtedly more experienced.

Source: https://www.cbr.com/tensura-rimuru-tempest-best-isekai-protagonist/

Diablo defeated Violet with ease in their duel, and Rimuru didn't even break a sweat defeating Lacua in a midair battle over Raja's castle.

Source: https://www.cbr.com/reincarnated-as-a-slime-crimson-bond-movie-criticisms/

In the course of their fight, Rimuru even had his arm cut off by Benimaru, though his healing abilities allowed him to regenerate without an issue.

Source: https://www.cbr.com/reincarnated-as-a-slime-rimiru-biggest-challenges/

It was bad enough that Rimuru made eating her food into a punishment for his people, as seen with Gabiru earlier on.

Source: https://www.cbr.com/anime-characters-who-never-eat/

Milim hoped to use her full strength, but Rimuru asked her to hold back somewhat.

Source: https://www.cbr.com/exciting-reincarnated-as-a-slime-victories/

Rimuru could easily survive any Grand Line island and convince other like-minded warriors to join him to form a new crew, perhaps called the Slime Pirates.

Source: https://www.cbr.com/tough-anime-characters-who-could-survive-grand-line-one-piece/

Shion is a cheerful deredere ogre who is evidently even more loyal to Rimuru than to her own tribe's prince, Benimaru.

Source: https://www.cbr.com/strongest-slime-scarlet-bond-characters/

That Time I Was Reincarnated as a Slime, Rimuru Tempest, is endowed with several powerful skills.

Source: https://gamerant.com/darkest-anime-abilities/

While Rimuru would ordinarily have little trouble in a one-on-one fight, Milim was even more powerful than Rimuru could handle.

Source: https://www.cbr.com/reincarnated-as-a-slime-rimiru-biggest-challenges/

While Rimuru would ordinarily have little trouble in a one-on-one fight, Milim was even more powerful than Rimuru could handle.

Source: https://www.cbr.com/reincarnated-as-a-slime-rimiru-biggest-challenges/