this monster seems good to be a type of mid boss

A good monster for a level 2 party.

Great way to whet your appetite for some vampires, eh?

Kinda concerned about throwing this thing at level 3 PCs. That's a good amount of HP, and multi-attacks to boot.

Bear mode activate, level two character. Creature seems unaffected.  Lol

Quote from Seydis >>

Kinda concerned about throwing this thing at level 3 PCs. That's a good amount of HP, and multi-attacks to boot.

With a challenge rating of five, this monster all by its lonesome is considered a medium difficulty encounter for a party of four level 5 PCs.

The DM's guide goes into gory detail in the Creating a Combat Encounter section, but I think they summed it up very nicely later in the Random Encounter section by saying:

"... The following monsters are considered appropriate combat challenges:

• A single monster with a challenge rating equal to or lower than the party’s level."

Quote from Seydis >>

Kinda concerned about throwing this thing at level 3 PCs. That's a good amount of HP, and multi-attacks to boot.

Our party managed to beat this at lvl 2, praise be the ghost of gary it missed every 2nd attack.

Monster Manual Pg 270

Hahaha, it took me way too long to realise that there was a person in the picture.

It seems like an Engulfed creature can not be removed with Rules as Written, compared to language used for Gelatinous Ooze.

Psst, over here, wanna be a truly evil DM?

Pair this with Will-O-Wisps and have them heal heal the mound 2d8 per wisp per turn, and instantly kill any party members that drop to 0 (on a con save of 10).

swamp thing 

 How does this monster damage? What are the weaknesses?

Quote from Rynegade >>

It seems like an Engulfed creature can not be removed with Rules as Written, compared to language used for Gelatinous Ooze.

 Yes, I think there's an oversight/omission in the rules text here. I'd suggest using the Gelatinous Cube's rules for escaping from an Engulfing shambling mound (though maybe with a higher DC).

Curse of Strahds death house

This needs a swamp variant where it has absorb fire and fire immunity 

Multiattack. The shambling mound makes two slam attacks. If both attacks hit a Medium or smaller target, the target is grappled (escape DC 14), and the shambling mound uses its Engulf on it.

Is the Engulf action meant to occur on the same turn as the two Slam attacks? The Multiattack wording seems to imply this, but with Engulf listed as a separate action I'm not so sure.

I just bought the miniature for this monster cause it was so cool and would love to run it for my daughters 3 person level 1 party instead of all the normal 1st level suspects < humanoid monsters but it would absolutely destroy them. Any ideas on scaling this back to be a somewhat scary and challenging monster for a beginner group? I was thinking that they could run into smaller " chunks" of the mound throughout the adventure then at the end after fighting all the little guys they see it come together and merge into one large mound ala a Transformers for the final confrontation. Thoughts?

Quote from Dwayne >>

I just bought the miniature for this monster cause it was so cool and would love to run it for my daughters 3 person level 1 party instead of all the normal 1st level suspects < humanoid monsters but it would absolutely destroy them. Any ideas on scaling this back to be a somewhat scary and challenging monster for a beginner group? I was thinking that they could run into smaller " chunks" of the mound throughout the adventure then at the end after fighting all the little guys they see it come together and merge into one large mound ala a Transformers for the final confrontation. Thoughts?

 Perhaps use some other plant creatures before, like an awakened shrub, or just use goblins that have vines wrapped around them, maybe coming out of their mouths and wrapping them up.