Did you ever find out the answer to this? I'm looking at exactly the same scenario (minus the specificity wrt classes).

A sick part of me really wants everyone to die from the wail right off the bat for comical effect as I say:

"Congratulations on your completion of 'The Lost Mine of Phandelver.'  I have been your dungeon master.  I hope you had a good time.  Excelsior!!"

And then just walk away.

Quote from Wolfe_Grim >>

@Trentos_Shadewalker

I know I'm a little late to this but, you could also add new features and abilities to make it a more challenging fight, such as adding abilities like

Phantasmal Grasp
Concentrated Physical Ability, much like a grapple but if the Dex Save fails is guaranteed to happen as the Banshee phases through your attempts to counter until her arms and legs wrap around your body, holding on to you. You are free to move but any attempt to remove her causes 1d10 necrotic damage as she tightens her hold and while any outward weapon will have a chance of hitting her, it is also likely to hit you as well.

Would give her very strong melee-ability against a level 5+ group especially if it was a Haunt (I think is the correct turn for something like this) of Banshees (ie a group vs group rather than group vs 1) or if you wanted her to be more ranged focused.

Corrupting Aura
Passive Trait, death and corrosion follow this being wherever they go, killing plants, sickening animals, and leaving a swath of eeriness and unease in their wake. Con Save success deals 1d4+1, Con Save fail deals 1d6+2. (does not affect undead or constructs)

Or just a block ability where she focuses on her etherealness and phases out of reality (essentially loses her physical form and cuts her movement speed in half or down to one quarter)

 These are fantastic suggestions. I'll be adding them to my repertoire in LMoP tonight. Thanks.

These things aren't that difficult, they will have very little issue BUT I was in a game where the entire party failed their save on her Wail so instant wipe lol

DM used this in our first campaign. Almost tpk'd the entire party. We only survived because I had relentless endurance. Love you dm Marko <3

In case this helps anyone, I used a banshee against my 3 level 3s (Druid, monk, and hexblade). Everyone immediately failed the horrifying visage saving throw which made things much harder. When she used her wail, the whole party failed and dropped to 0. I had her go back through a false wall since they were no longer a threat. Luckily no one failed all their death saving throws and one rolled a Nat 20, giving him 1 hp. Once they healed up, they summoned her by touching her things and after a long fight managed to kill her. 
I knew going into this that it might be a rough fight with a TPK. In the end they won with the warlock and Druid going unconscious once and the monk going down three times. 10/10 would recommend. 

I'm looking at the resistances of this thing and I'm wondering how a Druid might be able to even damage this thing???

The other party members would be a Warlock, a Fighter, a Rogue, and a Paladin.

Any tips would be very helpful!

Late to the party, but I think you should consider encounters without "safety nets".

Give warnings of course if they're up against something truly horrific (a good motto is: the bigger the threat the more obvious it should be; if there's a particular dangerous attack like a Catoblepas deathly gaze then some kind of warning like corpses with eyes burned out, or survivors telling tales of people dropping dead just at a look from the creature) and a heads up regarding DMing style (if they used to assuming all encounters are going to be balanced), but the PCs can decide for themselves if they need more prep. Obviously you're not out to kill the party, but maybe take the training wheels off and let them ride the bike. If they fall, they fall.

Just a suggestion!

1992 Space movie

This looks like a ghost

Quote from Trentos_Shadewalker >>

Looking for advice on this, as it would be the first time I use the Banshee.  Could a party of four, level 3 characters (Barbarian, Cleric, Wizard, Warlock) feasibly take this on barring bad rolls yadda yadda, as the climax of a story arc?  What are some DM's experiences with using these on lower levels?

The wail has a potential for a TPW, but I like the thought of putting these characters on the edge of their seat.  I might develop a safety net if its felt that four level 3's shouldn't be taking this on.

Any feedback would be great!

DM: Trentos

 

 In my experience, in the climax of an adventure, the boss' CR should be at least 2 levels higher than the average party level, and nothing in the banshee stat bloc would be incredibly unfair. the odds of everyone failing the role to beat the wail is very low, so the probability of a TPW is no higher than any other CR 4 boss. 

There's no easy answer - basically 4 lvl 3 characters "should" be able to defeat a banshee reasonably easily but there are risks. 

The large number of resistances and immunities can make it seem tough but if your party can throw the right kinds of attacks it can be a short fight (radiant, magic weapons, force for example), they're fairly low hp so even resistant attacks can take her down in a few rounds.

The incorporeal movement can be extremely difficult to deal with if the GM play hard - if your party knows how to hold actions etc. they should be ok, if they aren't strong on tactics and mechanics a tactically minded GM can swing well above it's weight.

The wail is the big problem as it could be a TPW, you can always provide them some small defence against it - e.g. a friendly Cleric gives them a death ward or you drop it when it doesn't catch the whole party - or drop it thematically to drive them away but retreat or taunt them to give them a chance to get back on their feet.  

In a straight, stand up fight with no wail 4xlvl 3 should have very little problem even if they only have elements the Banshee is resistant to

Just had a party of 8 each fail the Wail. Every single one less than 13 on their con save. The funniest and stupidest TPK I’ve ever seen.

My DM put four of these things against my party of four level threes and was surprised when we lost. 

Well I sent 4 of these to my 6 8th level characters and it almost took the party down. The thing with the wail is that it doesn't care how much HP you have.

Quote from Dr_Frederick >>

Quote from BillyBuster >>

What are its weaknesses?

 Mirrors, they hate those.  They’re vane, and don’t like being reminded of how monstrous they’ve become.

A generous DM might even let you reflect it’s Horrifying Gaze back at it.  Silver-backed are especially good for flavor.

 Cool.  The description was updated to include this information.

Well that wail thing's A Little overpowered...

Sam... what the hell

Sam.....wow wtf

Besides all information that were already given.

As with many other monsters, the difficulty of such an encounter is not only based on the stats. If the Banshee is played right, she should be easily be able to dominate the party in her home turf. Her abilities would give her good chances to surprise the party and take them out before the fight would have even properly started.

Ambushing the party with a wail and taking out others by fright would make a fight pretty hard for the rest, considering her resistances.

As a player, I would be extremely on my guard if I‘d enter the territory of such an enemy.

In a head on fight, face to face where only the initiative would derermine who starts the fight, they would be in a much better situation depending on their luck. But from my point of view, the DM would not do this monster any justice if he would play the encounter this way.