In 2e, traps are there specifically to slow down the party and force them to be cautious. Even in 5e, traps are a break in the action and can be triggered before anyone sees them. The 2e style doesn't sound like your playstyle at all, but even the way 5e uses traps by default seems to not be your style either. So...
Tell them the trap is there
Traps are interesting when players interact with them. Tell them the trap is there! Yes, 5e has Passive Perception for this purpose, but don't let that ruin your fun if giving the check away "for free" will significantly improve your and your players' enjoyment of the game. (Passive Perception is still useful for surprise encounters and ambushes, after all.)
By telling them that a trap is present without telling them exactly where and what, you get straight to the interesting interaction, as they investigate the trap to figure out where it is, what it does, and how to circumvent or disable it. They may play it smart or they may make mistakes and suffer the trap's consequences. Some traps will be boring (and quickly dispatched) and some will be super-interesting (and be super-interesting). Either way—smart or disastrous, trivial or super-interesting—you are keeping the part of traps you find fun (the players messing around with them), eliminating the part you find frustrating (caution bringing the dungeoncrawl to a literal crawl), and quickly moving along to wherever the fun is actually to be found.
So skip straight to the fun, and tell them that there is a trap here. Let them figure out what kind and where exactly, but tell them:
As you move to cross the threshold, your keen adventurer's survival instinct tells you something is wrong. There's something not quite right about either the door's stone frame, or the room's floor… or maybe it's something else nearby that's off. Anyway, what do you do?
...and cue the flurry of investigation!
DCs for the traps, should then be converted like most DCs in 5e. 10 for easy, 15 for moderate, 20 for hard, 25 for very hard.
I have now DM'd the final boss fight myself. So, as no one with direct experience has thus far responded to the question (though many of their suggestions were good), I've written up my own experiences, detailing what went well and what went badly. Hopefully this will be useful to the people who have favorited this question, some of whom may be currently DM'ing LMOP themselves.
What approaches have you used successfully in your own game to make the final encounter with The Black Spider 'memorable and exciting'?
- We'd played the entire campaign up to this point in the theatre of the mind. Suddenly revealing a full map and playing pieces for this fight definitely added excitement and grit, it also really helped to keep track of the AOE spells (faerie fire, web, darkness etc.) rather than me feeling like I was fudging who was hit and who wasn't.
- I decided to add hp to the Black Spider and turn him into a 'paragon monster'. As the final boss, I didn't want him going down in just two good hits, so I decided to give him two separate hit point pools of 37 hp each - when he went down the first time he 'regenerated' into a more dangerous final form (two turns per round) which really appealed to my players, familiar with boss fights in pc games, and made the Black Spider feel appreciably different to any boss they'd faced before.
What approaches have you used successfully in your own game to make the final encounter with The Black Spider feel balanced against a larger sized party?
- As mentioned above, I added extra hp to try and slightly balance out the action economy.
- I added two extra giant spider minions (bringing the total to six). Having as many spider minions as PCs definitely added to the drama and the spectacle but if the rolls had been more on my side this could have been a bit too much.
- We used milestone levelling, so I decided to keep the party at level 4 for this fight. Fifth level would have removed a lot of the danger, half of the party would have gained Extra Attack, not to mention the extra HP, spells and proficiency bonus increase.
- I treated the Black Spider as a fifth level caster (as opposed to fourth level), so that he rolled a second damage die for his cantrips.
What would I do differently if I did it again?
- The Black Spider is fairly toothless once all of his minions have been killed as most of his spells are crowd control, rather than damage dealing (only cantrips and magic missile, melee a last resort). If I was redoing this fight I think I'd give him the option of one scroll of fireball, to be used if the fight was going too quickly south.
- Make better use of webbing as difficult terrain in order to crowd control the PCs.
Best Answer
Quickly, I thought that my DM wrote this question, because we were in that exact same scenario. What helped us was:
The experienced players knew the material, and each took a pupil to help guide the new players. We're still having trouble actually "roleplaying" as a group. Most still treat it as a series of battles and not as an interactive story. We're getting better as a group, but be prepared for inexperienced players to decide that all they want to do is kill things. An overwhelming battle is a good idea for this. Make them need to retreat/consider not engaging.
Our DM used the total number of players as the guideline for us. Instead of 6 orcs, there were four orcs and two ogres. Instead of a group of skeletons, there were two wights and a high level mummy. Some caused problems, some didn't, but it was all fun.
Never underestimate the power of deus ex machina. An inexperienced DM can overwhelm his party. He can also use deus ex machina to solve the problem he created. For instance, we are a group of 6 (2 experienced players), and we had an encounter suitable for 8 characters. When it was clear we were in trouble (it was our 3rd/4th encounter), the DM used his god-like powers to help us out. We were near a town, and townspeople "witnessed our heroics and were inspired to help us" and took some flanking positions near some of the weaker enemies. This allowed our healer to retreat and heal the two tanks, and the rogue and ninja to clear out using sneak attacks. I think one villagers did 2 damage with a pitchfork. The damage wasn't the important part, but the attacks weren't centered on as many of the party, and the flanking helped with extra damage/increased attack rolls.
Just remember to think of creative solutions to problems that you will create, even just by mistake. In a dungeon, perhaps a trap springs and hits an enemy, rendering him unconscious for 5 rounds. Thing like this will allow the newbies to learn combat, and you can phase out the deus ex as you learn to better estimate your players skills and knowledge of the game.