Hello! I am a first time DM, and for my campaign, I thought it would be entertaining to have my players fight none other than the Philadelphia Flyers mascot, Gritty. The basic premise is this: they are trapped in a time loop that resets every time they die. Their only goal is to kill Gritty, whose stats I am still working on but plan on making border Terrasque levels of difficulty while still being killable by a 5 person level 20 party. Every time the players die, they level up, long rest, full reset. However, this means that Gritty also gets a long rest each time they die. Do you have any advice for how to keep them engaged until they are able to defeat Gritty, and any ideas for what kinds of abilities to give him? Thank you so much!

So, are you planning to start them at 1st level? Cause that will be ... difficult. They'll just get stomped every time until they hit somewhere in the low- to mid-teens when they might have a fighting chance. The first several times, however, they'll just die quickly and probably spend more time leveling up than they will in combat. Also, roguelikes (or if you want to call it a time loop) sound cooler than they actually play when it comes to D&D. I mean, you know your group, so maybe they'll enjoy losing a dozen or more fights in a row, with no hope of victory, but that's what's likely to happen.

If you go through with it, you'll probably want to give them gear along the way. But that, too will be less than satisfying. Instead of the standard play loop: We won, look at this cool stuff we earned through our victory, now we can go fight even more powerful enemies. The loop will become: We lost again, look at this stuff that just showed up through a combination of pity and DM contrivance.

Instead. I'd suggest just running it as a one-shot starting them at level 20. Or just running it as a traditional campaign, making Gritty the final BBEG, and they work their way up to him. Though as a first-time DM, I'd really suggest staying away from trying to run a 1-20 campaign. Start at 1, and plan on running it until it ends, don't set an arbitrary goal of it ending at 20; just have it last as long as the story lasts. Or better yet, take them through something like Lost Mines of Phandelver, which is a great way to learn how to run a game.

What level are they starting at?  I don't know if as a player I'd be enamored with fighting the same encounter possibly/likely 20+ times until the party gets it right.  You're right that engagement is going to be a problem.  

If it's a CR30 encounter like the Tarasque, and you want to do the time-loop thing, maybe instead of having them level up each loop, maybe start them at L20 and grant them an Epic Boon or a new Magic Item each time?

I don't think that the players are going to enjoy 20 battles against the same creature. It's just not fun. They'll know from level 1 that they can't win until much later, meaning that once they figure out the "fight, die, level up, repeat," they will decide that their best bet is to suicide into it time and time again to gain levels. At level 6, for instance, they already know that 1-2 hits are going to kill them regardless of what they do, and that they can't even hurt it. By level 6 I'd just be tossing down my sword and saying "Well, let's get this over with."

Honestly, I just don't think that this is a good idea for a game system because it (a) relies on redoing the same content over, and over, and over, and (b) the PCs can't actually lose because they don't ever really die. They are guaranteed to get to the level they need eventually. Even short combats in D&D will take more than 30 minutes with a party of 5.

You might want to consider that you could run a short campaign (say, 4 game) on a similar premise but make the monster CR26 with 800 hit points, and make the party level 10. Give the monster a bunch of immunities and a bunch of weaknesses, and put stuff in the world that lets the PCs try different strategies against it. At the end of each in-game day, they fight it and die and wake up again. The game then becomes as much about the preparation time they put in learning about it, finding its scat and checking out what it eats, learning from the cave paintings in that complex in the hills what the ancients did to defeat it and so on. They need to find the 4 clues, and at the end of each day they have to fight it. That might keep things interesting - no levelling up. This premise is only interesting if the characters strategies and preparations are what matter, not their stat increases.

Being repeatedly forced to go up against a monster you have no hope of defeating and against whom you make no progress sounds completely demoralizing, to me. The smart move would be either to, as Sanvael said, get yourself TPK'd as quickly as possible or simply walk away from the quest entirely for some variety.

What if instead Gritty wasn't a static CR 30 monster? What if he scaled with the party, growing in power as the party does? Every few levels, he comes back stronger than before and the party has to defeat him again. Perhaps he'll keep growing in power indefinitely until the party discovers and secures the source of his evil resurrection - the Stanley Goblet. 

Conversely, what if he stayed a CR 30 monster, but each time the party fights him, they discover or create new vulnerabilities and he grows weaker. This would allow for each fight to have purpose besides level grinding and would give player choices significance. If you did this, I would start the party at level 10 at the very least and make it clear what the campaign was going to be like. Players could easily get frustrated and quit if they don't know they're signing up for Live, Die, Repeat: The D&D Campaign.

I think a sense of purpose and progress would be important to something like this. Early fights should still have consequence other than "we get to level up when we die."

I have gotten stomped by something way over my head in a time loop campaign, and it wasn't much fun. It's pretty frustrating when your choices don't matter because there's no way to win.

I'd focus a lot on gaining info, learning about the creature, and in later loops making preparations based on what you've learned. Early fights should have achievable goals that a smart, resourceful party could complete before dying. And the knowledge or effect of those wins should play into future confrontations.

And I'd make the whole thing about 2 sessions long. As others have said, it will start to feel repetitive no matter how much you try to change it up. As a new DM jumping right into something unorthodox, by then you will likely be ready to either move on to a new idea or decide you'd like to get your DM bearings with something that requires a bit less homebrew work.