Master Level dungeons takes 50 stamina with 10 rounds. Basically the differences between Expert and Master are the damage and defense of the monsters. And the majority of spawns are Devils, which attacks per turn. So we need a really good strategy for encountering Master Level dungeons.
Rule #1: Fully Charge Up Your Monster's Skills
If you guys haven't noticed, the first round of (almost) all dungeons starts with only 2 monsters. This is always the chance to kill one down first and slowly eliminate irrelevant gems to get your monsters' skills charged up. That being said, you will always want to keep your HP at max.
Rule #2a: Sweeping
Every, but not all, special dungeons have a midpoint miniboss - usually at Round 5. If your team are strong enough, Round 2-4 can usually be "swept" by using up the skills slowly charged from Round 1. This approach could be risky when you reach Round 3-4.
Rule #2b: Repetitive Charging Skills
If you do not have confidence sweeping Round 2-4, all you have to do is to kill all monsters but one left using some of the skills. Then recharge the used skills.
Rule #3: Midpoint Miniboss
Miniboss is usually weak. This is always a good place to recharge up the skills if you haven't.
Rule #4: Round 9 Miniboss
Never try to sweep Round 9. Again, if there's two Liliths, kill one, and go back to Rule #2b.
Rule #5: Saving Gems
Using up too much attack gems can be troublesome, ending up not able to do anything except making big combos with no damage. So while you are charging up skills, you can move your gems to the bottom area of the screen at spots/patterns where they will not be eliminated easily, like the following two examples (please ignore the top half, the dungeon is irrelevant to this topic)
|Saving Wood Gems - irregular pattern|
And if you follow these rules or strategies with the correct team leader and subs, you should be able to tackle any Master Level Dungeons :)