D&D players kicking your baddies' butts? How to fix it FAST

"The Monsters Know What They’re Doing: Combat Tactics for Dungeon Masters" by Keith Ammann is a great book for NPC tactics. Detail about specific monsters as well as general stuff like "creatures with X level intelligence approach a fight like this."

Like others here mentioned, don't let your BBEG travel alone. 5e, especially, has ramped up damage in relation to hit points, and a full party will be dealing damage at 5 or 6 times the rate of any single monster. Add numbers to even things out. In groups, even minions can be a serious threat to an experienced party of adventurers. Also, I can't reiterate Ginny's final comment enough. If the party does something clever, don't punish them by not letting it happen. Some of the best stories later on involve a clever turn by the characters that blindsides the bad guys.

As you were talking about lair actions the first thing that came to mind was the baddy somewhat exasperatedly saying "dang it, this is really going to raise my insurance rates" or "man, and i really loved this rug" before unleashing an area of effect that would definitely mess up some of their own stuff (fire, water, etc). It would explain why they had waited until now to use it.

Can we please get a steal this side quest with this elf character? I really like her outfit, it looks amazing (especially the gloves), and I think she could have a good quest were the party encounters her, gets a quest or mission to stop her then goes to her home in a castle or something and you fight her there. I could be a fun way to get some new magic items or something similar.

One thing my DM did that I thought was clever: a surprise transformation. We were confronting a tyrannical Arch Bishop who had seized power. This fight had been built up to for half the campaign. We absolutely out-played him, separating him from his focus and giving him no way to fight back. The DM let us kill him, as we had earned it...but then the string-pulling BBEG showed up and turned the Arch Bishop's corpse into a monster. "Some minions are more useful dead than alive!" We ultimately ended the fight by defenestrating the monster-corpse, leading to it being impaled by a pole bearing the country's flag. And when the citizens saw it, one of them said "now he's as ugly on the outside as he was on the inside".

My favorite monster tactic when the big bad is losing way to hard? RUN AWAY! Any sane and semi intelligent creature would do this and it makes logical and narrative sense that if the villain is under prepared and caught off guard, they would high tail it to gather their strength and be better prepared next time. If the players find the enemy in their lair or stronghold there would likely even be trap doors or secret tunnels made for exactly that situation. I can be easier to just delay the encounter for another session and rethink your strategy, and if done well can ramp up anticipation a little bit more. A few things to note: give your players loot for driving off the enemy otherwise they'll just feel like you wasted their time. Don't pull this more than once or at most twice with the same boss or it will get annoying. Make sure to mix up the next encounter with some new abilities so it doesn't feel like a %100 repeat.

As part of thinking tactically for your boss, also remember: Scouting. The boss probably has spies, and scrying spells. As the characters run around using their schticks to take out monsters survivors and witness' can later report these to the boss who can then be prepared.

I found a great gauge for whether a monster, a big boss especially, needs beefing up is the player's reactions themselves. I had a group utterly crush (from my PoV) a BBEG in two rounds, taking very little damage and resisting everything the villain threw at them, but the players were super tense about it and feeling like every passed save or critical hit was an absolute clutch roll, and like they were dancing on the knife edge of the villain regaining control and sweeping them away. In that situation, I found it pretty much perfect to let the players have their speedy victory, because they felt like it had been really hard won, regardless of the reality.

As someone who played video game RPGs far before TTRPGs, one thing I like to borrow for bosses is the puzzle boss. Boss is average strength but is super resilient or untouchable because of X reason, and players need to survive and defeat minions while disabling the spell/item/etc. Once the boss is vulnerable, wiping the floor with them is very cathartic to the players

Tip: prime your players to accept these things as "normal" by building them in to low level encounters too. If they've seen it before during a relatively easy combat, it won't raise suspicions that you're changing things on the fly if (when) you need to use some of them to last minute alter the difficulty of a boss fight.

Literally the first "boss fight" I dm'd, I didn't think about one of the characters being a Paladin and my two undead dragon wyrmlings being undead - they failed any and all saves against being frightened and couldn't maneuver themselves in a position where at least their breath weapons would have been dangerous. The players felt very powerful though, so I'm not too mad about it :D

Simple, give them minions, meatshields or phases where the villain is invulnerable until the players perform a difficult task.

One idea of adding a Magic Item that the BBEG TOTALLY had the whole time is from MTG, Liliana's Chain Veil. The Chain Veil is an intelligent magical item that boosts the usurer's power but slowly drains their free will until the Veil is using THEM, instead of the other way around. It gives the BBEG an excuse not to use it ASAP, & leaves the party with a nice cursed item as part of the Loot for the next chapter.

I remember one of my first outings as a DM. I was looking at the monster I'd planned, and thought. "Oh no. This is too big and scary for a group of new players. let me shave off some hit points." Then combat starts, and I'm like. "Oh Wow! I underestimated your abilities. Never mind, regular HP is back in play."

"Definitely, totally had the whole time." This is a perfect place for red herrings to show up. For a villain being set up for so long, the players have been hearing about what they can do, how scary and powerful the villain is. And much of what they learn they can't be certain of what is fully true and what is rumor. Tell the players five or more things that are "false" that can be true if the villain is underperforming. Do this with every villain set up and the players have to worry if the villain is really all they seem. There's a part in the first Avengers that a friend of mine thinks is the same as this video's topic; An enormous snake-like flying creature shows up that's bigger than the buildings in New York. Hulk lands on it and takes it out in one hit. What does the GM immediately do? Summon two more. You want to kill my boss so easily? Deal with more of them XD Also, sometimes luck just isn't on someone's side. There's a "Leviathan Rule" that my first playergroup realized; The bigger the enemy, the faster they die. Coined because every single time (and I mean EVERY SINGLE TIME) a leviathan has shown up, it has somehow died before it got a turn and often on the first player's turn. At this point, it's just how things work, no different than saying that red potions always heal you or that every metal is magnetic in this setting.

Saving this to my D&D playlist because it might come in useful once I start DMing. But also, my group's DM doesn't need to worry about this because we're the kind of idiots who fight a ghost on unhallowed ground, barely escape with our lives, break into the mayor's house to get weapons instead of just asking, accidentally give the same ghost a massive power-up and nearly get kidnapped by witches who barely had anything to do with any of this. That ghost was supposed to essentially be the tutorial villain.

For monster strategy, major recommendation to read "the monsters know what they're doing" outlines great tactics and motivations for a wide range of enemies.

One of my favorite D&D moments is when we dispatched a purple worm super fast (I got a hold monster on it early in the first round) and so the dm just had a second purple worm crash down in through the ceiling.

I had one single bard and one single paladin turn what felt like an impossible battle with a lich into no problem when the lich failed a roll that let the bard's phantasmal force hurt him with, I shit you not, 'radiant kittens'.

RE: adding items. Beware that anything you give to the enemy, the players can potentially acquire for themselves. Be careful not to give them something tooooo overpowered.