The triggered ability is odd. It means that if you cast it before combat, you'll get a double copy of the bonus as a oneoff. In later turns you'll never be able to match up to the bonus you got the turn you cast it, and I think that'll feel unsatisfying.

The Idea is that you will play this the turn you plan to Alpha Strike -possibly the turn after you cast Flash of Brilliance on your opponent's end step.

Which is why its second ability hits all attackers, not just yours.

Changed the ETB triggered ability to giving creatures Vigilance which still encourages you to play this before your combat step which was the concept - this guy/gal swoops down, fills the troops with Valour and then directs the charge, providing inspiration and tactical insight.

Unfortunately, it's an Angel and, in the spirit of equality, all attackers get the second ability boost, every turn, even your opponents

I don't like it being 6/6 (it doesn't need to and white shouldn't get too many good large creatures), but I think the biggest issue is the last ability helping opponents. Not because I think mythic rares should be all upside (Abyssal Persecutor is a good example), but because I think +2/+0 plus first strike makes it horribly difficult for you to block and survive even the tiniest of creatures.

Cut it to 4/6 and attackers get +2/+0. It is specifically meant to help everyones creatures but you are right , with first strike it is crazy.