What awful RNG... broke my 6-0 arena run.
Went 2nd, against a Mage, and had 2 flamestrikes in my deck. Had one in my starting hand, but decided to mulligan it because all my cards were 4+ mana, and I felt I needed a couple 2-3 mana cards to not sit there the first three turns and end up with under 20 health before I play a single minion (and chances aren't bad one of the 4 mulligan cards would be a flamestrike back, or if I kept it, I could end up with both).
Of course, the 2 flamestrikes don't draw until literally the last 5 cards in the deck, would have won if I had drawn one of them in the first half of the deck and didn't have to use them on single creatures when we were topdecking and trade horribly with minions before then (also had the pleasure of having my only Arcane Intellect in the last 5 cards too). Guess the rule is no matter how awful your hand is, never mulligan a Flamestrike, even if you have a ton in your deck.
I actually don't play as Mage much in arena, so I had to learn that one the hard way. I guess even 3 flamestrikes in your hand before turn 7 could maybe be a good thing, they'd never count on the third and fourth after all.