I know folks had to have done this before so I'm looking for some advice. 

I want to run a one-shot where the DM is the BBEG.  Basically, the players wake up with amnesia and I hand each one a character sheet.  They play as that character going through some of the most iconic D&D battles and puzzles looking to "save the land".  As cliche as I can manage without being super obvious.  They get to the tower of the BBEG and finally come face to face.  In the tower is a room filled with half-constructed maps and minis and dioramas, all of the battles they've gone through.  The BBEG is sitting at a desk, surrounded by notes and binders and crumpled paper and it's the DM.  

Dialogue, role playing, etc.  Then they fight.  The DM obviously has crazy power but fights within the rules they have been following for the world.  At some point they trigger their special ability and force the players (who fail whatever save) to hand over their character sheets.  The DM shuffles them and hands them back out.  

Anyways, I want to make a realistic DM monster who isn't actually impossible to defeat.  Like, it should be very hard, but doable.  Any suggestions on ways to make this fun and not frustrating for the players?

Also, what are the most cliche, traditional, etc monsters and/or puzzles you know of?

To answer your last question first: have the party start in a tavern, obviously. The traditional band of goblins is a great go-to. For puzzles, there's always the locked door with a simple riddle you found on the internet written above it, where the solution is like "water" or "the sun" or something like that. 

For fighting the DM, hear me out: what if the combat begins with like a modified lich or dragon or something like that, but at a certain point the DM reaches their true power and becomes invincible, and the way they have to beat it is by physically stopping you, the DM in real life, from playing anymore. Like they have to get up from the table and steal your dice and statblock from you. Could be fun and meta, while still having a real bossfight using their character sheets and stuff before.

DM as a boss sounds really fun. you can reflavor legendary resistance as "fudging the roll for narrative purposes" and maybe midfight, minions appear, looking hastily drawn onto the map like paper jam dipper. Traits on the statblock that can give you instant advantage as a reaction. maybe a shifting battlefield that progresses through all the areas the players have fought on during the campaign. 

as for cliche monsters: Treasure chest Mimic can't be beat for cliche.

Oh, I kind of love this idea.  And my players would absolutely be game for that.  

Yes, perfect!

Quote from Flouatiousyop >>

Anyways, I want to make a realistic DM monster who isn't actually impossible to defeat

Well, that's the issue, isn't it? A "realistic DM monster" would be impossible to defeat

In other mediums where this kind of story gets told -- Chuck Jones' Duck Amuck, Grant Morrison's run on Animal Man, the USS Callister episode of Black Mirror, etc. -- "victory" doesn't usually involve winning a big fight. I'd figure out what would actually count as "victory" for the party here. Maybe it's solving some sort of puzzle that frees the 'forever DM' from their own prison. Maybe the party just has to escape the world where they are the playthings of a power-mad DM

Keep in mind too that if you really want this to be a one-shot, you won't have time for a bunch of combat, so a non-combat solution at the end probably works better anyway