A Grimward Archer is a spectral monster that only appears in chapter 3 of the Shadows of Amn campaign.
If you have sided with Aran Linvail in chapter 3 — when you undertake the third task for Aran, two of them, along with two Grimwarders, are encountered in the Lower Tombs guarding the entrance to Bodhi's hideout. Two more are also found within Bodhi's Lair along with two more grimwarders and three Fledgling Vampires.