IMO, anything with less than 100 HP x party level is a speedbump in this edition. Monsters are pushovers as written.
Ran a fight this past weekend with a 5th level Paladin, Bard, Thief and Warlock. The boss had 450HP. 18 AC, resistance vs cold, fire, acid. Magic resistance and imposed disadvantage on spell attacks, and if both attacks missed or it made both saves, it took no damage and healed 10HP. Each round it could, as a bonus action, toss out 2 commands or summon d4 spinagons or dretch. 6 attacks. At every 150HP it lost one of it's aspects (so lost 2 attacks, and one of it's bonus action options), then uttered a howl which automatically frightened everything in 30' radius for 1 round and forced a DC 16 con save or be paralyzed (save ends) and granted the creature DR 10 vs all attacks for 1 round.
To my surprise, 0 party deaths, but I did almost get the bard. Would have if a spinagon had rolled decent on either of its last 2 attacks. 2 back to back crits for 60+ damage on the paladin's part helpde move things along. The had some lucky rolling and a decent ambush strategy, but still... this is the kind of thing you need to do if you want a set piece battle. Ignore RAW, and play to your party, not some hypothetical group of chumps they must have playtested on. IMO, the MM throws such softballs you really just have to go outside of the recommended exp budget if you want to make it memorable.
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