So, i have wasted more than 2 refills without a single drop meaning it could take more than 20 refills to complete this task. Sounds like a bug to me and if it isn't, please submit a request to change the requirements. That is ridiculous.
Again, just comes down to RNG unfortunately. My best suggestion is to do higher stages when you can to make it less likely the smallest potions will drop.
I can pass this along but I doubt anything will change.