I don't see how you guys can think this is a high level campaign. Basic enemies off the top of my head are Imps, goblins, humans, skeletons, gnolls, & spiders. None of those scream level 5 or higher...ever
You forgot phase spiders, the matriarch, hag, redcaps, bulette, minotaurs could all be considered level 5 and up. The only reason they can be destroyed fairly easily is due to Larian combat.
In a straight up 5e ruleset fight, some of those encounters would be very challenging at levels 1-4.