Next Time, maybe Against the Giants?

It's from 1979/1981, redone for 5e in "Tales from the Yawning Portal".

5e VERSION by WOTC

"Tales from the Yawning Portal". Page 166. 

Reddit says way too easy: https://www.reddit.com/r/dndnext/comments/9z7mi5/running_against_the_giants_5e_and_my_players_are/

       Oh sure. 12 is way too high for AtG - it's challenging at like 9, maybe. Even the book claims it's for 4 L11s, and they balance around 4 people who actively make the worst possible characters and have never heard of tactics. And it's a bummer, because it's built around stealth and subterfuge, and it's pretty fun if you play that way, but if you *can* win the fights, then it's just a tedious slog of unending combat.

When I ran it for overleveled PCs, I replaced every damn giant with their beefed-up version from SKT. Go nuts. Tucker's Kobolds those giants up, add traps, have them rush to each others' aid, give them all spellcasting. Make the remorhaz invisible, reskin a roc as a giant peacock, bump up the dragon an age category, throw in an ice devil on a diplomatic mission. As written, it's going to be a cakewalk, so feel free to pull out all the stops.     

REDUCE PARTY's MAGIC ITEMS IF NEEDED: find in-game mechanics to deprive them of their items. This isn’t as hard as it sounds. Ethereal Filchers and anti-magic fields were made for just this reason.     

       I’ve never found hoards to slow things down as much people seem to think, I’ve ran encounters with 30+ enemies and my turns are still faster than some of my players. Obviously use group initiatives, sometimes I’ll divide enemies into 2 or 3 groups either by type or location. You can have enemies arriving as rounds go on as well. Basically, before enemies turns decide which/how many attack each player and then just move pieces and start rolling dice on your turn, dice rollers are helpful here. Have the players AC so you quickly determine how many hits each and dice roller up the damage. Don’t do too many enemies with spellcasting or lots special abilities, most of the enemies should be mooks with just an attack or two. Some people use swarm rules, but personally I always found that a bit off. I do however have groups of archers do area of effect attacks sometimes.

But if you just want to buff enemies, try and make them more dynamic rather than just packing on HP, damage, or AC. For powerful inidividual enemies, legendary actions are a good way to accomplish this. For example, give remorhaz 3 legendary actions, 1 point for a slam dealing 3d6+7 bludgeoning+3d6 fire, 1 point to swallow a grappled creature, and 2 points to disengage and use it’s burrow speed. Now, it can engage your frontline then dive away go for the wizard, makes it much scarier. Mounts, flybys and cunning actions are also good ways to make enemies a bit more shifty.

Lastly, consider terrain and tactics in your encounters. Archers are good filler monsters, archers behind cover with some difficult terrain in front of them who focus fire on the wizard are even better. Have enemies try control the players, use choke points, scatter traps around, grapple and shove. Spellcasters are great for this, Silence would be vicious on your all spellcaster party, but even lowly fog cloud can used to cover enemy movements and keep your players on their toes. Good, challenging encounters are hard, I struggle with all the time, but things all have helped me. Good luck to ya!     

AD&D VERSION CONVERTED in 2016, not by WOTC:

https://www.dmsguild.com/product/189455/Classic-Modules-Today-G123-Against-the-Giants

AD&D VERSION, converted to 5e stats unofficially  

They say the AD&D version converted (before the official 5e conversion in 2017) is mega hard, but also say it's a legendary campaign circa AD&D: https://www.enworld.org/threads/against-the-giants-for-5e.491175/

Mega hard b/c fireball in AD&D did 85% of HP of a Hillgiant. In 5e, it does 25% of HP of a Hillgiant. And giants hit harder in 5e, like more than dbl the dmg. 

"I am currently running AtG, and they've made it to the Hall of the Fire Giant King. The biggest issue I've run across isn't the deadliness of the encounters, but the lack of room for the number of creatures. In AD&D they assumed Giants only took up a 10' square, but in 5E they take up 15', and that extra 5' make a big difference for combat and just fitting in the rooms(no pun intended). I've had to change tactics and move/remove creatures just to get things to fit. If you are concerned about the deadliness, then removing monsters will help with this issue as well.

I would say level 9-10 is a good starting point, and try to get them to level about each adventure. Since you have an experienced player, it shouldn't be hard to remind them that these are HARD adventures designed to test the Player's (not Character's) skills. I think letting the players know that stealth is a far better strategy than charging in is almost a necessity (my group had to argue a lot with the Barbarian PC to keep that from happening)."

"I think this is a common mistake when comparing editions. The role of a 1e magic-user was to blast monsters with high damage spells. The 5e wizard only fits that role as an evoker, other schools are more about controlling the battlefield or making other characters more effective."

This.

I say run it as-is, as a one-shot. If the players lose, they lose and can try again with a different approach. The intermediate TPKs will make victory all the sweeter when it finally happens.

I can think of combat-heavy approaches that would likely succeed (e.g. heavy use of Polymorph; or a party of Moon Druids (Pass Without Trace, Spike Growth) with a Necromancer 6/Warlock 3 who freely uses Rope Trick to let the party rest anytime and anywhere), and there are more strategic approaches that would likely succeed too (Combat As War; decoys w/ illusions and high mobility to make the enemy divide his forces so you can defeat them in detail). Eventually the players will find something that works for them."