Just one more thought: that person down at rank 76 who is at boss level 500 has had a nearly perfect run.
The minimum amount of damage you can have done to be showing up as boss level L+5 is
(L^2)/5 + 9*L - 180
So in my screenshot above, I am at boss level 175, so my L would be 170 — that’s the highest level I’ve beaten so far. Plugging that into the formula, the minimum amount of damage I can do is 7130. Since that’s the exact amount I’ve been credited, you can see that my run has been perfect at these relatively low levels. (Note that this will almost surely change, since my next fight is against 3 Tomes of Evil.)
If you’re appearing at level 500 on the leaderboard, you have definitely beaten level 495. The minimum amount of damage you can have done is 53280. The person down at rank 76 is at boss level 500 and has done 55,205 damage. That’s just a handful of losses, which would increase your damage dealt but not your boss level. Possibly just one loss, since they could have beaten boss level 500 already.
So, good job @MickiTheFox!