I was running the Lost Mine’s of Phandelver adventure module as DM with my group of mostly 1st time D&Der PCs. Party was made up of a Dwarf Fighter and Elf Cleric (a married couple IRL), a Tiefling Barbarian, a Gnome Rogue (my wife) and an Elven Wizard. The wizard wasn’t a regular, only coming once every other or so session, so the party was frequently short a decent caster, and the wizard, when present, never prepared and was frequently at a loss as to what spells to cast. He needed a lot of hand-holding, which is the last thing you want in a wizard.
Anyways, when the core group hit Level 4, my wife decided to multi-class with a level in Wizard, so they could have some basic spells to utilize when necessary. The session that first saw this multi class put into action was when the party was heading into Cragmaw Castle to try and find Gundren Rockseeker, so they could locate the map to Wave Echo Cave. Somehow, the party located the entrance to Cragmaw around the back of the building, and the 3rd room they enter contained the bugbear, King Grol, and Vyerith, the Doppleganger.
After quickly defeating the Bugbear, the Doppleganger shapeshifted into an exact replica of the Tiefling Barbarian. Everybody failed perception to try to see if they could tell any differences between the actualy PC, or the doppleganger. My wife, the L3 Rogue/L1 Wizard decides to cast Tasha’s Hideous Laughter on the doppleganger. I had her roll a percentage die 50/50 to determine which barbarian to hit. She hits the doppleganger, and it fails its save. The doppleganger immediately bursts into a fit of laughter, and can no longer shift into anything that will disguise it, as it will continue to laught for the duration of the spell. The party immediately kills it.
The party then crushes the rest of Cragmaw castle, as it defeated the bosses immediately after entering the castle and continue with mostly full HP and expendables.
They also did this in the Redbrand Hideout and Thundertree. They happen to take the easiest, shortest path to the endboss and take it out immediately, then carouse the rest of the dungeon with full stats. The DM life ain’t easy…