So, I've been roped into assisting with an event at a local game store where I hold regular sessions. 

The premise is max 50 players, each with 3 Level 10 characters i.e., 3 lives (no flying races, shapeshifter and other limitations) ... 6 DM's where each represents a deity on a plane of existence that test the players ala Hunger Games... PVP, PVE stuff where the champions of each plane go on to the final labyrinth.  I partly feel in over my head as I've not run games for level 10 characters... I'm still somewhat new to being a DM but I like a good challenge. I had strong "tomb of horrors" inspiration when I said yes but I am unsure of how to make something actually killer but not frustratingly unsolvable and I don't want to copy it of course.  My grid is also limited to 22x24 (5' squares).

We know 5e is mostly designed to let the players win. I will have maybe 7 players at a time, and I want to throw some crazy deadly monsters at them. I happen to have an army of undead stuff mini wise that I would like to use, but I know I need a variety to keep all classes in check. So, I'm looking for suggestions.

I am asking the community here for any and all advice, ideas, whether I should bow out or rise to it and do it.  

You could run some deadly puzzles, that will give them a challenge they may not have built their character for. 

Level 10 isn't a broken level by any means, it's worth checking through the spells they are likely to have access to so you know what to expect (EG Fly can trivialise many an encounter).

For some interesting encounters, consider challenges in which hte monsters are obstacles. This way you can give them more powerful foes, and if they choose to just fight it, it's on them that it was beyond their abilities.

For example, you could have a ritual which needs to be stopped, with tasks to complete in a labyrinth to stop it. If they fail to stop it, they die. If they fight the minotaur, they (probably) die, and if they do live, they have no time to stop it, so they die. 

Also time limited tasks are a good one to make it a challenge. I have a trap called "Your soul on my wall" which has the party trapped inside a painting. They start off looking around to work out how to get out. Then they notice their skin feels oily, and smears like paint. Then they realise what's happening, and start to panic. On all the walls, there are pictures of the room they are in, with groups of people painted in various poses - hammering the frame, curled up in a ball, digging through the desk. None of them are moving.

That one can trap/kill a whole party without a single combat dice rolled.

What you need is a copy of Grimtooth's Traps.

CRs should be probably in the 15 to 20 range, single monsters at a time, but as soon as they finish one another pops up. Go with three, then have a period slightly longer than a short rest, then a period of traps and puzzles, and then another three monsters, then back to puzzles, and so forth.

Make one of the puzzles a bunch of riddles with time limits on answers -- and make it real time. Like "you have thirty seconds to answer this riddle." Miss the riddle, take damage.

I mean, this is a deathtrap, so each step of the way you let them know ahead of time that someone must die. "each part of the cycle, one must fall, and one can return. If the tests do not do it, then you must."

That reinforces the PVP part and might encourage them to give wrong answers and work less together.

You could do a hall way with a impossible to kill monster (i.e the Demogorgon or something) that forces them to dash and run as he walks slowly behind. Anything that attacks or slows down is just dead. That one tentacle monster from ACQUISITIONS INCORPORATED for arms of a bigger creature.

I'm not sure I understand the format of the competition.

There are 50 players and 6 DMs. Is this groups of 8 and 9 players each?

Do all the players compete simultaneously?

Each DM represents a single plane and the winner of each plane goes onto a second round.

This seems to me that there can only be ONE winner in each contest which means the other 7 or 8 characters in each group have to be killed off. On top of that each player has 3 characters so the intent seems to be to make this incredibly deadly so that only one player is successful at the end while everyone else has used all three of their own characters.

If that is the case then your question about what to run is actually very difficult to answer because the characters are competing against each other and the environment to be the last one standing. If the environment is too nasty then there won't be anyone standing at the end.

If I was running something like this, I might go for a maze with extra dimensional space rooms at some of the vertices of intersecting corridors to be combat arenas. The maze would have 16 entrances, 4 on each side - draw from a token pile for starting door. Players would not place tokens on the board until they encounter someone else. When the two players enter one of the rooms where corridors join, the first one is stuck there, the second that enters becomes their opponent. They fight until one wins - maybe use a separate 40'x40'x40' room for the fight. The loser respawns at a maze entrance if they have a character left and the winner can continue on their way.

Add some traps and wandering monsters in the maze to stop someone from trying to hide or avoid combat.

Just an idea but might be interesting.

Maybe you could do something akin to the high sun games, with a labyrinth that shifts around the party for specific challenges. Example have it box them in and then release an enemy for them to fight in order to progress through the labyrinth, and  so on and so forth  for other puzzles with the goal to beat the other teams to the center and progress to the next round.