Q:

In the opening song, who is Rimuru fighting in the town?

In the opening song of That Time I Got Reincarnated as a Slime, between 1:13 and 1:29 of episode 8, there's a group of people who appear to be knights of some kind and the slime is fighting what looks like the best guy, but these people are never shown in the anime.
Who are they?

A:

If you mean the bunch in white clothes that are shown right after the Demon Lords Milim, Frey, Carillon and Clayman, and just before the red-headed guy (Guy Crimson), those are the paladins of the Holy Church, the most predominant religion in the Western Nations.
Most of them have yet to appear even in the current official translation of the Light Novel. Only their captain, Hinata Sakaguchi, has made an apppearance so far. She first showed up at the end of volume 4, and if I remember correctly she's shown at the end of the first season of the anime as well, but doesn't take any action unlike in the novel.
So no, they won't appear in the anime, I guess they were added to the OP just to create hype among the LN readers.

