Seems like Belkar's foes are using 'minion' rules.
Na, most are probably just 1st level experts or warriors. Belkar is about 12th level ranger/barbarian and I'm sure his daggers are magical. He's enraged and he's got Mr. Scruffy helping him do damage. All that stacked up and they don't stand a chance.
Hell the caster used Acid Arrow, Bull's Str and Scorching Ray (all 2nd level spell). Plus he did sneak attack. So at minimum he's is 7th level. The bard did inspire confidence +2 so she's between 8th and 14th level. But everyone else is probably very low level members of the group.
Where did it point out that it was inspire courage +2? She mentioned using Inspire Competence for a +2 bonus, but that can be done at level 1. (Tasha's Hideous Laughter would require at least level 2).
Based on Varsuuvius's spellcasting, he's 14th or 15th level, so the rest of them should be that too.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.