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
You are looking for sniped being helped by an auxiliary be verb. Do not confused with snipped, which is the past participle of snip.
He was sniped by an unknown combatant.
Note that sniped can also be used in reference to the person sniping. But you won't need the auxiliary verb in most instances.
He sniped the enemy from a very long distance.
Assasinate means the target was specifically chosen.
to murder (a usually prominent person) by sudden or secret attack often for political reasons