Thanks for playing!
The boss only appears on the Very Challenging difficulty or higher, and it will spawn when you reach level 6 or 7.
The number of tokens you earn at the end of a run depends on several factors:
The fewer potions you drink, the better.
The less damage you take, the better.
The more enemies you kill, the better.
The higher the level you escape with, the better, up to level 15 which is the max.
The more credits you escape with, the better. The score is capped at 100,000 credits, but if you manage to escape with more than that, you'll earn even more tokens.
Good luck.