thanks for the feedback, I was going for endless minions and regenerating boss (who spawns minions) - so the need to kill boss is the primary goal. I made it so without dying / upgrading killing boss is near impossible (at 1-2 deaths).
I would have like to add more variation of enemies and perhaps some lead up to the boss. The stink aim was a little intentional but I would have liked to include some more forgiving and fun upgrades like beam and mines.