aand still not got it, is this crap bugged or what?? never seen anything like this over 40 kills on the final level and not 1??

I assume by large you mean the Greater Potion (the middle one)?

I suggest using stage 7 or 8 for such mission, they have larger drop rate on the middle potion.

It's not bugged because I just did lvl 15, and they drop fine, but they do droo the Superior one much more (the largest one) and kinda hard to drop the greater ones.

It happens to all of us don't worry, not a bug, just bad drop rate

na not the middle one, its the big one the last one :P and i got one in the end but thats insane

 Always use the lowest available stage for these missions usually drops pretty 

Personally if I spend over 100 energy to get the superior potion and I dont get it, I do something else for awhile and come back and its usually the 1st thing I get. 

When I get in a snag like that I try different levels.  8-15 once of each.  

I am also hung up on the FORCE MISSION to get 1 Superior potion, minimum Level 10.  I have tried levels 10 through 15 several times, with no troop loss if that matters.  I got everything else, lesser and greater potions, beer mugs, shards, etc but no Superior.  It is holding up what appears to be a whole freight train of other missions but until I get past this one, it is all for naught.

Spending more hundreds of energy for this seems rediculous when there are so many other things to do.  It is possible that Red Superiors come your way by other means but until you do this way it is a mess.

How do I even know it is working correctly?

It is aggravating, I know.  But I do not see how spending money, aside from getting more energy, helps your chances of getting the Superior potion.

You'll need to spend hundreds of energy on potion keeps often enough once you start ascending champions to the higher levels. If you have something time-sensitive that you'd rather be doing now, then ok, do that instead and come back later. But it's not abnormal that you have to spend hundreds of energy before getting a superior potion, even on stage 15. It's RNG - another time you might get lucky and get a bunch of them in just five runs, or so. Would be nice to have stages 16-20 on potion keeps like you have in dungeons, with harder battles but better potion drop rates, but I guess that's not a priority. 

I revisited the mission again, running from L10 to L15, but Lo and behold I hit a cluster of 3 Superiors at L13!  So that one is done...but now they want me to get a Superior for VOID lol.

I switched my focus on Force cuz I want to get to L3 scension on Relickeeper, that needs lots of medium potions, both red and white.

I am less concerned about ascensions cuz thse potions can be obtained in various other ways, but  being tied to some specific challenge/mission is what makes it tougher.