Side A has bows and arrows, swords + limited magic. Side B had only bows and arrows, swords -- but has recently developed explosives and primitive guns (probably in part imported from another land, where the tech is being developed). Does all that make sense so far?
And is it reasonable that they would be evenly-matched for some years to come? Discuss.
It sounds like the Native Americans and the European colonists / invaders. Even with side B adopting firearms it didn’t turn out well for them. Combination of population mass and ruthlessness.