im going to be dming a one shot in a week for 3 people excluding myself that i am making from scratch, and need help with what levels and encounters to do

the general idea i have is that they die at the start to the bad guy, then have to escape the underworld (which i will just make as a dungeon rather than an actual entire realm) before the bbeg is able to start the end of the world

im planning for the game to be split over 2 sessions, and my 3 players are all relatively new but have all played the LMoP starter kit thing which was my first time dming a while ago. 

Any suggestions for what level i should have them as for a demon based dungeon of mid difficulty and a decently hard final fight would be greatly appreciated 

thanks in advance

You should start by looking at what type of Fiend (Demonic) creatures you want to populate your dungeon.  From that point, you can go back to the player level to work on setting the characters.  The truth is, Fiends have a wide variety, with CR going from 1/8 to 30, so it is more what you want to throw at your players.

Being reasonably new, I wouldn't go for too high a level - you know what your group is experienced with, if you jump to a high level then they may spend half the oneshot trying to learn their characters or skimming through books to root out abilities that they're sure they had, rather than enjoying the game!

For how challenging a oneshot should be, that is entirely up to the theme of the oneshot. Some oneshots are political intrigue or mysteries, where the players might do a great job unfolding it or they might fail. Other oneshots might be more a case of how quickly they can solve the mystery, rather than how difficult it is. You need to decide what the challenge is going to be.

Assuming this will be majorly combat, you will be dealing with resource management - your players should try to gauge how much of their abilities they need to use on each combat to make it through to the end. To this end, it's a good idea to make the first encounter about representative of the rest they can expect, with a more dangerous one at the end.

use the encounter calculator as a guide, not a rule - they say 6-8 encounters per adventuring day, so if you plan that many and make them hard encounters, then it will be a hard oneshot, if that's any guidance. Take it with a pinch of salt, and try to gauge your players - if they are more powerful than expected then up the combats a bit, and vice versa.

The key thing about a one-shot is that, if you're running a game with a duration of six hours or something, people don't want to spend two hours creating their characters and learning what they can do. This usually means low level characters. Not necessarily first level, but probably something level 1-4.

As for difficulty: depends what you're after. You can afford to make it really hard, since a TPK is just ending the session a bit early, but that depends on whether your players would find that annoying.

Quote from Scottsketches >>

the general idea i have is that they die at the start to the bad guy

 

 This is unsolicited and maybe not even necessary depending on how you were going to handle this, but I've seen this mistake made before so I gotta say - if you are planning this as an actual combat where you wipe the party with a creature way out of their league, don't do it. It's not fun for anyone but you. Since this essentially happens before the real game starts, narrate it. You can have fun with it and let the players describe how they attack or even how they die, but you don't need to play it out and they should know right off the bat that they die.

For me, 3 is the magic number for a one-shot. At least until you and your group are more experienced and maybe become interested in trying some higher level builds. Most classes have a decent amount of customization by level 3, so your guy isn't just the generic fighter #3 that he was at level 1.