You have to assume you will be spending a few stones the first time you try to beat these high end dungeons.
I know some people refuse to spend stones for dungeons but you have to look at it as an investment. It's not like a $5 REM roll that most of the time gets you crap. Hera-Is will be a 40% drop rate chance for the easy version. You will be getting the standard free stone every day for a week, which seems to be every other week now. These bosses are set up to make you spend money. Just looking at the boss ability list for this dungeon I'm like yep, this is a 3 stone fuckfest for mythical. I don't plan on surviving those mass bind abilities. I know I am going to die and then will spend a stone to pop back unbound at full health. But when she dies on mythical I get a 100% drop of a boss I know I will use.
Then I will do it again because I want 2 for my blue team.
With all the free stones we keep getting every other week my total has been around 30 for the last few weeks without spending any cash. It's quite nice.