How do I clear the Production floor in Fortune 499

The unexplained rule of the puzzle is that if a monster X is standing on the spawner × (its starting position) when the summoner & is defeated, then all monsters will be killed on the same turn (as hinted at the start of the floor).

So the solution becomes merely:

(1)            (2)            (3)            (4)           

+---------+    +---------+    +---------+    +---------+   
|@#.......|    |.#.......|    |.#..↓←←←@|    |.#.......|   
|↓###.###.|    |.###.###↑|    |.###↓###.|    |.###.###.|   
|↓.X...X..|    |.X×...X.↑|    |.X×.↓.×X.|    |..X...X..|   
|↓###.###.|    |.###.###↑|    |.###↓###.|    |.###.###.|   
|↓###.###.|    |.###.###↑|    |.###↓###.|    |.###.###.|   
|↓.X...X..|    |.X×...X.↑|    |.X×.↓.×X.|    |..X...X..|   
|↓###.###.|    |.###.###↑|    |.###↓###.|    |.###.###.|   
|.........|    |@→→→→→→→↑|    |....↓....|    |.........|   
|.........|    |.........|    |.........|    |....@....|   
|....&....|    |....&....|    |....&....|    |....&....|   
+.--------+    +.--------+    +.--------+    +.--------+   

Go down the    Then up the    Then down      Fight the     
corridor       corridor       the middle     summoner. 

This floor is not the end of the level and a number of boss fights come next. If you lost a lot of health on this floor trying to figure a solution out, I would advise you to take an intentional death and $100 hit (or loading from your save file) so you can start the next floor with enough health to win.