It is all RNG and at level 15 your chances are not too good. I assume that this is for a challenge or mission. Honestly, you are probably better off putting your effort elsewhere and maybe running this 5-10 times when it is open. It is always good to have extra potions lying around. 

I will let others more knowledgable that I comment but that is my $0.02.

I will continue because there should be some from time to time at this level normally. And when he's consumed my 1300 starting energy and I still don't have any superior potions, I'll deem the nonsense to be complete and quit the game.

So ... don't farm 15? There's a reason that they added the higher floors?

bruh do you at all remember how it was when you had garbage champs ? 😂 its a beginner mission he probably cant farm higher than 15, its not anime, you cant beat stronger guys with the power of friendship

Yes I am lvl 48

The progress missions accompany us in our evolution (it is one of them that requires the 5 potions from lvl 13)

It's good to be limited when you don't have the level (here to farm level 13 or +) but there I just ended up wondering if the machine (RNG) was working correctly: I spend all the energy that I had saved to harvest shit in addition to wasting my time

Thought I'd test this out, did 15 runs each on Magic,Arcane and Force keep at level 16. Won 5 superior in Force, 6 in Arcane and 0 in Magic. Can't do Spirit atm but just shows what RNG is capable of.

Dont try so hard.  Do 10-20 per day.  RNG is working.  You will get those potions.  It may take longer than you expect.  

This game is designed to frustrate you so that you spend money to make it easier.  But you can progress without spending.