Probability is probability. If it's not 0% or 100% then there are no guarantees. A particular piece of loot could have a 90% drop rate and you could still go a hundred runs without seeing it, and nothing would be wrong with the system.

The particular minion you want will probably never be added to the cash shop, but it's possible it could some day be a mogtome event item. For now, all you can do is continue to run the dungeon or wait for someone to board it a price you're willing to pay.