Yeah - a lot of what these guys have been saying
Absolutely, this player is using an effective sniper tactic - there's nothing wrong with his assessment of the rules, he absolutely can do that as a mage and it works and the Shadowrun universe knows it works (hence every window being mirrored and important things are underground)
Like many powerful tactics - like sniping with a rifle, throwing explosives,hacking their systems, full automatic fire - there are times when it will work and work well and times when it will be useless or next to useless. When I run a game with a min maxer my creed is always to let them succeed sometimes and throw complications sometimes just as I would at a decker, sam, face or any other role - highly specialized builds often suffer out of their comfort zone - my players never have charisma 1 not because I forbid it but because sooner or later someone will have a conversation with their character and react badly (I mean, sometimes they have charisma 1 but they know it's going to affect their game and not just be a cheap dump stat that never comes up)
- Spotting targets - a 1 km unobstructed view to a target might not be all that common, any indoor, in vehicle, behind a building, around a corner and it's immediate fail. How's the weather? How good are those binoculars? Get some binoculars, go try spot a target at 1km in any town - flying helps but streets have houses on both sides, if you even know where you're looking - go up a high building and try to spot a particular address - it's really not that easy, time to acquire target is much longer than if you're up close, re-positioning for a different angle takes much longer - actions to observe in detail before every spell to spot targets, more actions to identify them (as I would also expect from a rifle sniper at those ranges) - Oh, is that a ward - guess I'm done here
- Getting spotted - So, flying 500m over a city, doesn't anyone have radar? do they have drones? do they have astral perception? Law enforcement might get interested, especially if lightning starts flying about, invisible? doesn't affect radar, doesn't affect ultrasound, doesn't affect astral perception - that spirit just looked up - oh, there you are, the only metahuman sized aura in a clear line of sight - that makes you much easier to spot than your target is who is covered by all that ground clutter.
- Response - well, any mages in the target area can throw spells right back, counter inbound spells, spirits, aircraft/drones, sniper rifles - if the PC is throwing direct energy (lightning, fire etc.) then he's lit up like a christmas tree, throwing indirect is a little stealthier (e.g. mana bolt) but has a bit less oomph, Watch any TV that has a sniper shoot - as soon as they are aware a sniper exists everyone puts cover in the way - behind a vehicle, behind a rock, behind a building, pop smoke, no line of sight means no shot
- The law - if a team go into corp territory the city cops don't care what you do. If a runner is in city territory throwing spells into corp territory then you have trouble with city security and corp security. Spells leave signatures behind, the higher the force the longer the signature hangs around. Throwing kilometer long lightning bolts over a city might peak someone's interest, it sure isn't subtle, that signature and that MO will end up on file with KE, with Lonestar, with Eagle security, being loud isn't always a good idea
I'm not saying it should never work, but just like any sniper there are loads of limits and counters and restrictions - at some point he's going to run into some of them
PS - just for fun, background count can be really entertaining when people are levitating.