How to make monsters rise from the ground?

Just like in Scythe 2, how monsters will randomly come out of the ground in front of you. How do you do this, does it require scripting?

Make an illusio-pit, put a monster in it, and use instant-raise floor special to raise the pit's sector instantly in order to make it look like the monster is appearing from out of nowhere.

lower a sector but don't texture the sides, then use linedef action 19 to instant raise

Use the Transfer Heights command to make the floor appear to be solid. Then, use a Floor_RaiseInstantly command to make the monster pop up. If you only feel like using the floor technique, that's okay, but you could always have the monster spawn, too.