In my experience, running through the level killing everything to the section just below where the boss spawns (the bit that goes down the ramp to the left) is enough to get the boss to spawn.
On the second try on each attempt I cleared the entire floor. That didn’t work on the second occurrence.
I also will note that logging out to the character screen and coming in did NOT seem to fix it. I actually had to quit the game and restart.
If this is a matter of killing a certain amount of enemies, maybe a counter can be added. I actually sometimes feel the same way on those Echoes that say kill enemies to find Boss Bad Guy. But it’s not nearly as bad on those because the count needed seems to be a lot lower than what they level actually holds.