Is there a verb that characterizes being shot by someone from a hiding place, perhaps at long range, like a sniper?
snipe is exactly the verb you are looking for. My Chambers dictionary gives two shades of the verb, neither has any hint that it’s a slang or childish word.
1. to shoot snipe [the bird] for sport
2. to shoot at individuals from a position of (usually distant) cover
3. to attack or criticize, especially from a position of security
to pick off by rifle fire from (usually distant) cover