Actually Atroce can be killed using the SW tanking technique. He doesn't attack fast enough to make a lvl 10 SW disappear while SG is hitting him. Just cast a new SW right after the SG ends hitting, recast SG and move to the next SW. Sometimes the spawns can get a couple of hits on your SW making it disapper while SG is still hitting or while you're casting a new SW so keep some fly wings ready for such situations.
That said, it's probably a lot easier just to use quagmire, SG and keep running. Just run in circles around the SG and you probably won't get hit at all.
Sorry going offtopic... This was supposed to be about snipers, not wizards. xD