Please feel free to correct me if I'm wrong, but I believe a high impact sniper rifle is a one-hit kill with a headshot to a Stormcaller. I've always taken down a Stormcaller with a headshot from my 1000 Yard Stare every time. Sometimes I've heard them activate it and I just wait for them around a corner and I've been able to take them out with one shot without them taking damage from any other source.