Rewatching the episode, the rooftop spot seems like the perfect place to set it up. Walt could have used it himself or just hired somebody. Why not?
Sniping is a skill. One that takes many years to master. The show has shown many times that Walt is not very good with a hand gun, so it would be very unlikely he could use a sniper rifle.
And finding someone he could trust to hire with this skillset would be very difficult, if not impossible in the limited time frame. He doesn't have the connections to the underworld that Gus has. Even if Saul "knows a guy who knows a guy", he would still have to set up the meeting, and make the plan all before the window of opportunity arises.
I happen to be binging through Breaking Bad, and on S4E7, Walt actually asks Saul about hiring a hitman. I can't recall the exact quotes, but Saul mentions that he knows two guys. Both of which know Mike because Mike is the one that referenced them. And if, "And that's a big if", they could find a guy, he'd still have to contend with Mike who seems to know how to prepare for everything.
So basically, Sniping was out of the question.