Do the waves of enemies at the end of a level ever stop?

Solution 1:

Um...

• After you grab the crystal, the game spawns mobs endlessly (UNLESS you've managed to gather enough dust to power EVERY room then NOTHING ever spawns!).
• Endless points early game where it doesn't spawn the "module killer" crystals and you setup your dungeon with enough turrets. (Points don't really matter but why not? My current run is an Easy Science Pod on floor 6 with 22,000 points. On floor 5 I killed 12,000 monsters just for funsies lol.) You can get away with this even on "Module Killer" floors if you have enough repair, though it's much riskier and requires a lot more attention, of course.
• Most importantly: Endless Food if you can manage the above scenario while also utilizing a character who gets +0.2 Food Per Kill (Soylent Green). Yes that's right get your free, endless food today!
• In addition to food you'll really be min-maxing with free, Endless Industry if you also utilize Recycling in your farming setup. :D
• Since monsters only drop dust if you are lucky and have a Merchant who uses dust for currency you can clear them out of items! This is good to do even if you have full loot on all your characters because you can then sell the extra loot to the merchant on the next floor for bonus materials/dust. :)
• Oh yeah almost forgot there is an Achievement (I play on Xbox One) for surviving IN the exit/elevator for 1 minute. ;)

My current team for this is: Gork (Solyent Green at Level 3), Skroig (SG at Level 3 and SG2 at Level 11), Nanor (SG at Level 7), and Opbot (Recycling at Level 5 and Recycling 2 at Level 12). In the farming room (ideally 5-7 small modules with 2-4 Autodoc, 1 Defense, 1 Tear Gas, 1 Attack) I give Nanor the crystal in floors that need repair, otherwise I have Opbot hold the crystal so every single kill nets food and industry.

Doing this I can achieve hundreds of industry and food each floor. Though some floors just don't work...you absolutely need a single hallway leading from your farming room to your exit, with no monsters spawning on that "side" of your farming room.

Just make sure to have a good exit strategy to get everyone out of the room when you need to, like saving Skroig's Red Plume skill to distract all the enemies at once. :)

Edit: Took a screenshot for y'all. You can see I'm on Floor 9 (on an Easy Armory Pod run) with way more industry than I'll ever be able to use (I still had over 1.5k at the completion of this run despite filling pretty much every room I could with stuff), and my heroes are already way stronger than they should be ehehe. :)

Also I used an additional defense minor module instead of an attack minor module because I was able to fill a number of other rooms with Tactical Huds.

Solution 2:

• After you grab the crystal, the game spawns mobs endlessly.
• Since monsters only drop Dust (which is useless at that point) and you don't gain exp for killing, there is no practical reason for fighting once you've reached the exit with the crystal.