LoS can be a little finicky. Most of the time if I can't see the enemy, they can't see me. But be aware that archer units seem to have pretty decent Perception, often enough, so their sight cones are fairly broad. Also, archers have a lot of AoE options, and they don't have to be able to see you directly to hit you with that. Without knowing what is actually hitting you, I'd only suggest you see if you're getting killed by direct hits, or AoE. As far as them spotting you in the first place, you know full well how touch-and-go Stealth can be. Is Invisibility an option for you?