Thanks so much for this man! I'm new when it comes to being a Dungeon Master!

Our party of 2 died to one of these tonight. It was a large size that blocked the escape from a dead end hallway in the Mines of Phandelver. We didnt know that it could split or that it was immune to lightning. There goes those characters.

y

It is saying you could 9 damage or 2d6+2 damage... 9 is the average damage

oof

What is the best cantrip or spell for level 1 Warlocks to defeat this jelly?

Quote from yuchenwei345 >>

What is the best cantrip or spell for level 1 Warlocks to defeat this jelly?

 When asking what the best cantrip for a Warlock is in a combat scenario, the answer is usually Eldritch Blast.

Am I crazy or 2 of these versus a lvl 1 party is INSANE!!! (about to start GMing Dragon of icespire peak) Even one would be deadly...

I dont think you're ready for this Jelly 

I've DM'ed a couple of groups through Dragon of Icespire Peak. Some groups had already done Umbrage Hill, but sometimes this was their first encounter/combat situation. Sometimes some members of the party were rendered unconscious with a single hit. I then typically let the 2 archaeologists run in to drag hurt people outside, so they at least didn't die. 

After running it a couple of times, and looking at some Youtube videos of people describing the lore, combat tactics, ... , I've come up with this solution:

The ooze likes to engulf a victim and 'eat' it (dissolve it by its acid). Instead of doing a pseudopod attack, I allow the jelly to perform a grapple (with advantage because of its size and fluid nature), which only does the acid damage (1d6) and grapples the player. On subsequent turns, players can do normal saves to break from the grapple (athletics or acrobatics). On the second turn a player is grappled, they become restricted as well (cannot attack anymore, because they're inside the jelly without freedom to move hands around). Every round they take 1d6 acid damage. 

Other players can try to break a captured creature free by using slashing/lightning damage (jelly splits open to release the creature).

I've got some more nuances like trying to isolate party members, dragging them off to a quieter place to feed, ...

Besides a more versatile way of fighting (remember, for many it's one of the first combat experiences in DnD, so you want to make it exciting, and not just hit / take hit), this also allows a DM to get rid of a single hit with 3d6+2 total damage to a 1D6 per turn. As an added bonus it really promotes teamplay, as it's a lot easier to free someone from the outside than it is to escape from the inside.

Sorry for the long post, but I struggled a bit with this myself to make it a worthy and fun battle, so hopefully this can help others.

deez

Sorry, I'm a year and a month late, but unless a damage is listed under "resistances" the monster is not resistant to it at all. 
It is not resistant to slashing damage and takes the full damage of the hit. However, it will split if it has enough HP to do so, and if it does, it technically takes no damage as it's health gets set to 22...both of the new Jellies, get that.

Damage Immunities Lightning, Slashing

It takes zero damage, even from a vorpal sword... it just splits.

Pseudopod. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) bludgeoning damage plus 3 (1d6) acid damage.

What does the "+4 to hit" mean? Does it have to roll 4 more than the target armor rating to land a hit or is this referring to something else?

smells funny. perfect aesthetic of molten peanut butter and jelly

Quote from merosario >>

Pseudopod. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) bludgeoning damage plus 3 (1d6) acid damage.

What does the "+4 to hit" mean? Does it have to roll 4 more than the target armor rating to land a hit or is this referring to something else?

 It means you add 4 to the attack roll result. (I.e. the jelly rolls a 7 to hit and you add 4 making the attack roll a total of 11)

I fought two ochre jellys with my dad and brother on my first campaign and they were way too powerful because of their immunity to slashing damage, which was a problem because my character was a dwarf with two battleaxes. the dm then nerfed them after unexpectedly introducing me to death saves.

Dragon of Icespire Peak has two of these up against 3 level 1 players - I feel like my players are going to get destroyed? Do you think I should only give them one?

Yes I had a party of 3 lvl 3s and with one having a giant wolf spider as a mount (gnome ranger with beast companion) which was included in the attack choices. The ranger got downed after splitting it with his short sword. The druid went down after that thankfully the third party member was a full plate cleric but they ran through all the spell slots.

Quote from BlueJayBuh >>

Dragon of Icespire Peak has two of these up against 3 level 1 players - I feel like my players are going to get destroyed? Do you think I should only give them one?

 Yes I had a party of 3 lvl 3s and with one having a giant wolf spider as a mount (gnome ranger with beast companion) which was included in the attack choices. The ranger got downed after splitting it with his short sword. The druid went down after that thankfully the third party member was a full plate cleric but they ran through all the spell slots.