Does it have negative priority or does it only work if the user is slower?
Metal Burst accounts for the damage dealt by the foe's last attack. If your Pokemon is slower than the opponent, and it uses Metal Burst, it will deal damage that turn. If your Pokemon is faster than the opponent and uses Metal Burst, it will fail provided the opponent hasn't yet used a damaging move. If you use it again next turn, it will deal damage based on the attacking move your opponent used last turn.
Is the damage calculated as 1.5 x damage of the move last used on it or 1.5 x HP lost?
It's calculated as 1.5x damage dealt.
Metal Burst (Bulbapedia)
