How will you handle "Replaying Completed Stages?"
Or will the Dimension Guide have the encounter recorded and sends the player in a pocket dimension based on the encounter?
demon magic bullshit lets you replicate previous fights you have had for training and loot reasons, its not real but its also real but not real
I've not yet entirely settled on a specific explanation, tbh, because there are a lot of options.
At one point, I even considered actual time travel, because that's literally a canon thing that comes up in multiple games
and is technically allowable under Worm physics, too.
Admittedly, that doesn't really explain how in the games your repeat fights don't necessarily use the exact same parameters as the first clear, though. Particularly, many of the games don't let you repeat boss kills, there's always some other stand-in unit that replaces them after the first time, usually to prevent you from farming the boss' gear.
Lately, though, I've been more inclined to do something like Zero G's suggested — in essence, you've got some function of the demon gate (and probably one the data record NPCs) that "records" fights, and then creates an item-world like simulation you can keep repeating.
Alternatively… I've considered just disallowing repeats unless I can figure out a way to make it humorous. Largely so that the story doesn't get bogged down in Gamer Syndrome where the readers spend 20 chapters reading about the cast grinding for stats.
I'm already going to need to find a way to avoid that as it is, though fortunately for those I have a ready-made solution through Netherworld Research & Item World Expeditions — with a few tweaks, they're an easy way to handle grinding happening in the background.