On the issue of Baer Serce using archers, I think we might be restricting ourselves to ingame references. Couldn't it stand to reason that the Serce do have a knowledge of archery? Why can't they have bows? On top of that, I wonder if they could have had a bit of an overlap in territory into the Ermine, where, perhaps, they had a better relationship with the fir'bolg than most other nations - maybe it was a sort of double entente.
What relation might the Serce have with the (former) kingdom of Yer-Ks and the Dire Marsh?