Two mana for a weak Impulse that puts excess cards in the graveyard is perfectly playable, and flashback gives us value in the late game if we mill this through other means
Be it a reanimator deck that needs self-mill, a storm deck, or a Splinter Twin + Pestermite combo deck, blue is usually involved somehow
But youre playing a burn/Fling deck
At the beginning of each end step, if a card left your graveyard this turn, Gau deals damage equal to its power to each opponent
If you get three different kinds of energy using Phoenix and then attach a fourth energy for the turn, you are already hitting for 100 damage, which is pretty solid for an attacker that appears out of nowhere and sets up in one turn
With Gardevoir ex and Mew VMAX [Fusion Strike] both having a Darkness-type Weakness, this is a great inclusion