Just out of curiosity, Master rune of Valaya says to cancel all dmg assigned during battlefield phase during the turn. Does this include any spell damage as well, such as flames of tzeentch?

I don't have the card in front of me, but if that's the way it is worded, then yes. It will cancel any damage assigned during the battlefield phase regardless of source.

That's correct, all damage done during the battlefield phase is canceled.

You can see the card here: deckbox.org/system/images/whi/cards/1-25.jpg