Can Marshadow GX's "Shadow Hunt" Ability allow you to attack on your first turn using Pheromosa GX's "Fast Raid" attack if Pheromosa GX is in the Discard Pile?
Yes, it can because going first is part of the Fast Raid attack that is being copied.
Source: TPCi Rules Team (2018-08-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_137
SM3_156
SM3_80
SMP_SM59
SMP_SM59